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  AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON SEPTEMBER 8, 1997
                                                      Registration No. 33-44851
=============================================================================== 


                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                               -----------------

                                 POST-EFFECTIVE
                                AMENDMENT NO. 1
                                       TO
                                    FORM S-8
                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933

                               -----------------

                       PIONEER NATURAL RESOURCES COMPANY
                     (AS THE SUCCESSOR ENTITY TO MESA INC.)
             (Exact name of registrant as specified in its charter)

             DELAWARE                                       75-2702753    
 (State or other jurisdiction of                         (I.R.S. Employer 
  incorporation or organization)                       Identification No.)

                               -----------------

                           1400 WILLIAMS SQUARE WEST
                         5205 NORTH O'CONNOR BOULEVARD
                              IRVING, TEXAS 75039
          (Address of principal executive offices, including zip code)

                              --------------------

                      1991 STOCK OPTION PLAN OF MESA INC.
                            (Full title of the plan)

                                MARK L. WITHROW
                           1400 WILLIAMS SQUARE WEST
                         5205 NORTH O'CONNOR BOULEVARD
                              IRVING, TEXAS 75039
                                 (972) 444-9001
           (Name, address and telephone number of agent for service)

                                    copy to:

                               ROBERT L. KIMBALL
                             VINSON & ELKINS L.L.P.
                           3700 TRAMMELL CROW CENTER
                                2001 ROSS AVENUE
                            DALLAS, TEXAS 75201-2975
                                 (214) 220-7700

         THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE
OR DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT
SHALL FILE FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(A) OF
THE SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(A),
MAY DETERMINE.

================================================================================
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                                EXPLANATORY NOTE

        On August 7, 1997, MESA Inc., a Texas corporation ("Mesa"), merged with
and into the Registrant (the "Merger") pursuant to an Amended and Restated
Agreement and Plan of Merger (the "Merger Agreement") dated as of April 6,
1997, by and among the Registrant, Mesa, MESA Operating Co., a Delaware
corporation and a direct wholly-owned subsidiary of Mesa, and Parker & Parsley
Petroleum Company, a Delaware corporation. Pursuant to the terms of the Merger
Agreement, (a) the Registrant assumed each outstanding option to purchase
common stock, par value $.01 per share, of Mesa ("Mesa Common Stock") granted
under the 1991 Stock Option Plan of MESA Inc. (the "Plan"), and (b) the assumed
options became exercisable for common stock, par value $.01 per share, of the
Registrant ("Pioneer Common Stock") at the rate of one share of Pioneer Common
Stock for every seven shares of Mesa Common Stock. The Registrant is filling
this Post-Effective Amendment No. 1 to the Registration Statement on Form S-8
to reflect the succession of the Registrant to Mesa, the assumption by the
Registrant of each such option to purchase Mesa Common Stock and the conversion
of the options for Mesa Common Stock into option for Pioneer Common Stock in
the Merger.


                                    PART II
               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT


ITEM 3. INCORPORATION OF DOCUMENTS BY REFERENCE.

     The following documents have been filed with the Securities and Exchange
Commission by the Company and are incorporated herein by reference and made a
part hereof:

     (a)  The Company's Registration Statement on Form S-4 (File No. 333-26951)
          dated June 27, 1997; 

     (b)  The Company's Current Report on Form 8-K dated August 7, 1997; and

     (c)  The description of the Company's Common Stock, $0.01 par value per
          share, contained in Item 1 of the Company's Registration Statement on
          Form 8-A filed with the Commission pursuant to the Securities
          Exchange Act of 1934 (the "Exchange Act") on August 8, 1997.

     All documents subsequently filed by the Company pursuant to Sections
13(a), 13(c), 14 and 15(d) of the Exchange Act prior to the filing of a
post-effective amendment that indicates that all securities offered have been
sold or that deregisters all securities then remaining unsold shall also be
deemed to be incorporated by reference herein and to be a part hereof from the
dates of filing of such documents. Any statement contained in a document
incorporated or deemed to be incorporated by reference herein shall be deemed
to be modified or superseded for purposes of this Registration Statement to the
extent that a statement contained herein or in any other subsequently filed
document which also is or is deemed to be incorporated by reference herein
modifies or supersedes such statement. Any statement so modified or superseded
shall not be deemed, except as so modified or superseded, to constitute a part
of this Registration Statement. Upon the written or oral request of any person
to whom a copy of this Registration Statement has been delivered, the Company
will provide without charge to such person a copy of any and all documents
(excluding exhibits thereto unless such exhibits are specifically incorporated
by reference into such documents) that have been incorporated by reference into
this Registration Statement but not delivered herewith. Requests for such
documents should be addressed to Pioneer Natural Resources Company, 1400
Williams Square West, 5205 North O'Connor Boulevard, Irving, Texas 75039;
Attention: Secretary, (972) 444-9001.

ITEM 4. DESCRIPTION OF SECURITIES.

     Not applicable.

ITEM 5. INTERESTS OF NAMED EXPERTS AND COUNSEL.

     Not applicable.

ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

     Article Twelfth of the Restated Certificate of Incorporation of the
Company provides that the Company shall indemnify its officers and directors to
the maximum extent allowed by Delaware General Corporation Law. Pursuant to
Section 145 of the Delaware General Corporation Law, the Company generally has
the power to indemnify its present 


                                      -2-
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and former directors and officers against expenses and liabilities incurred by
them in connection with any suit to which they are, or are threatened to be
made, a party by reason of their serving in those positions so long as they
acted in good faith and in a manner they reasonably believed to be in, or not
opposed to, the best interests of the Company, and with respect to any criminal
action, so long as they had no reasonable cause to believe their conduct was
unlawful.

     With respect to suits by or in the right of the Company, however,
indemnification is generally limited to attorney's fees and other expenses and
is not available if the person is adjudged to be liable to the Company, unless
the court determines that indemnification is appropriate. The statute expressly
provides that the power to indemnify authorized thereby is not exclusive of any
rights granted under any bylaw, agreement, vote of stockholders or
disinterested directors, or otherwise. The Company also has the power to
purchase and maintain insurance for its directors and officers. Additionally,
Article Twelfth of the Restated Certificate of Incorporation provides that, in
the event that an officer or director files suit against the Company seeking
indemnification of liabilities or expenses incurred, the burden will be on the
Company to prove that the indemnification would not be permitted under the
Delaware General Corporation Law.

     The preceding discussion of the Company's Restated Certificate of
Incorporation and Section 145 of the Delaware General Corporation Law is not
intended to be exhaustive and is qualified in its entirety by the Company's
Restated Certificate of Incorporation and Section 145 of the Delaware General
Corporation Law.

     The Company has entered into indemnity agreements with its directors and
officers. Pursuant to such agreements, the Company will, to the extent
permitted by applicable law, indemnify such persons against all expenses,
judgments, fines and penalties incurred in connection with the defense or
settlement of any actions brought against them by reason of the fact that they
were directors or officers of the Company or assumed certain responsibilities
at the direction of the Company.

ITEM 7. EXEMPTION FROM REGISTRATION CLAIMED.

     Not applicable.

ITEM 8. EXHIBITS.

     Unless otherwise indicated below as being incorporated by reference to
another filing of the Company with the Commission, each of the following
exhibits is filed herewith:

     4.1   -- 1991 Stock Option Plan of MESA Inc. (filed as Exhibit No. 10.19 to
              the Company's Registration Statement on Form S-4, File No.
              333-26951 and incorporated herein by reference)

     5.1   -- Opinion of Vinson & Elkins L.L.P.

     23.1  -- Consent of KPMG Peat Marwick LLP

     23.2  -- Consent of Arthur Andersen LLP

     23.3  -- Consent of Coopers & Lybrand L.L.P.

     23.4  -- Consent of Vinson & Elkins L.L.P. (included in its opinion filed
              as Exhibit 5.1 hereto)



                                      -3-
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ITEM 9. UNDERTAKINGS.

     The Company hereby undertakes:

          (1) to file, during any period in which offers or sales are being
     made, a post-effective amendment to this Registration Statement:

               (i) to include any prospectus required by section 10(a)(3) of
          the Securities Act of 1933, as amended (the "Securities Act");

               (ii) to reflect in the prospectus any facts or events arising
          after the effective date of the Registration Statement (or the most
          recent post-effective amendment thereof) which, individually or in
          the aggregate, represent a fundamental change in the information set
          forth in the Registration Statement; and

               (iii) to include any material information with respect to the
          plan of distribution not previously disclosed in the Registration
          Statement or any material change to such information in the
          Registration Statement; provided, however, that paragraphs (1)(i) and
          (1)(ii) do not apply if the information required to be included in a
          post-effective amendment by those paragraphs is contained in periodic
          reports filed by the Company pursuant to section 13 or section 15(d)
          of the Exchange Act that are incorporated by reference in this
          Registration Statement.

          (2) That, for the purposes of determining any liability under the
     Securities Act, each such post-effective amendment shall be deemed to be a
     new Registration Statement relating to the securities offered therein, and
     the offering of such securities at that time shall be deemed to be the
     initial bona fide offering thereof.

          (3) To remove from registration by means of a post-effective
     amendment any of the securities being registered which remain unsold at
     the termination of the offering.

          (4) That, for purposes of determining any liability under the
     Securities Act, each filing of the Company's annual report pursuant to
     section 13(a) or section 15(d) of the Exchange Act that is incorporated by
     reference in the Registration Statement shall be deemed to be a new
     Registration Statement relating to the securities offered therein, and the
     offering of such securities at that time shall be deemed to be the initial
     bona fide offering thereof.

          (5) Insofar as indemnification for liabilities arising under the
     Securities Act may be permitted to directors, officers and controlling
     persons of the Company pursuant to the foregoing provisions, or otherwise,
     the Company has been advised that in the opinion of the Commission such
     indemnification is against public policy as expressed in the Securities
     Act and is, therefore, unenforceable. In the event that a claim for
     indemnification against such liabilities (other than the payment by the
     Company of expenses incurred or paid by a director, officer or controlling
     person of the Company in the successful defense of any action, suit or
     proceeding) is asserted by such director, officer or controlling person in
     connection with the securities being registered, the Company will, unless
     in the opinion of its counsel the matter has been settled by controlling
     precedent, submit to a court of appropriate jurisdiction the question
     whether such indemnification by it is against public policy as expressed
     in the Securities Act and will be governed by the final adjudication of
     such issue.

                                      -4-

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                                   SIGNATURES

     Pursuant to the requirements of the Securities Act of 1933, as amended,
the Company certifies that it has reasonable grounds to believe that it meets
all of the requirements for filing on Form S-8 and has duly caused this
Registration Statement to be signed on its behalf by the undersigned, thereunto
duly authorized, in the City of Irving, State of Texas, on the 8th day of
September, 1997.

                                        PIONEER NATURAL RESOURCES COMPANY


                                        By:     /s/ Scott D. Sheffield
                                           -------------------------------------
                                                    Scott D. Sheffield
                                           President and Chief Executive Officer

     Pursuant to the requirements of the Securities Act, this Registration
Statement has been signed by the following persons in the capacities and on the
date indicated.

Signature Capacity Date --------- -------- ---- /s/ Scott D. Sheffield President, Chief Executive Officer and September 8, 1997 - ------------------------------ Director Scott D. Sheffield (Principal Executive Officer) /s/ M. Garrett Smith Senior Vice President--Finance and September 8, 1997 - ------------------------------ Assistant Secretary M. Garrett Smith (Principal Financial Officer and Principal Accounting Officer) /s/ I. Jon Brumley Chairman of the Board September 8, 1997 - ------------------------------ I. Jon Brumley /s/ R. Hartwell Gardner Director September 8, 1997 - ------------------------------ R. Hartwell Gardner /s/ John S. Herrington Director September 8, 1997 - ------------------------------ John S. Herrington /s/ Kenneth A. Hersh Director September 8, 1997 - ------------------------------ Kenneth A. Hersh /s/ James L. Houghton Director September 8, 1997 - ------------------------------ James L. Houghton /s/ Jerry P. Jones Director September 8, 1997 - ------------------------------ Jerry P. Jones
6 /s/ T. Boone Pickens Director September 8, 1997 - ------------------------------ T. Boone Pickens /s/ Richard E. Rainwater Director September 8, 1997 - ------------------------------ Richard E. Rainwater /s/ Charles E. Ramsey, Jr. Director September 8, 1997 - ------------------------------ Charles E. Ramsey, Jr. /s/ Arthur L. Smith Director September 8, 1997 - ------------------------------ Arthur L. Smith /s/ Philip B. Smith Director September 8, 1997 - ------------------------------ Philip B. Smith /s/ Robert L. Stillwell Director September 8, 1997 - ------------------------------ Robert L. Stillwell /s/ Michael D. Wortley Director September 8, 1997 - ------------------------------ Michael D. Wortley
7 EXHIBIT INDEX
Exhibit Description of Exhibit ------- ---------------------- 4.1 -- 1991 Stock Option Plan of MESA Inc. (filed as Exhibit No. 10.19 to the Company's Registration Statement on Form S-4, File No. 333-26951 and incorporated herein by reference) 5.1 -- Opinion of Vinson & Elkins L.L.P. 23.1 -- Consent of KPMG Peat Marwick LLP 23.2 -- Consent of Arthur Andersen LLP 23.3 -- Consent of Coopers & Lybrand L.L.P. 23.4 -- Consent of Vinson & Elkins L.L.P. (included in its opinion filed as Exhibit 5.1 hereto)
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                                                                    EXHIBIT 5.1


 (214) 220-7700                                                (214) 999-7700


                              September 8, 1997

Pioneer Natural Resources Company
1400 Williams Square West
5205 North O'Connor Boulevard
Irving, Texas  75039

Ladies and Gentlemen:

     We have acted as counsel for Pioneer Natural Resources Company, a Delaware
corporation (the "COMPANY"), in connection with the filing of Post-Effective
Amendment No. 1 to the Registration Statement on Form S-8 (the "POST-EFFECTIVE
AMENDMENT") which amends the Registration Statement on Form S-8 (File No.
33-4485) (the "MESA REGISTRATION STATEMENT") of MESA, Inc., a Texas corporation
("MESA"). The Post-Effective Amendment is being filed with the Securities and
Exchange Commission (the "COMMISSION") in connection with the assumption by the
Company of certain options to purchase common stock granted pursuant to the
1991 Stock Option Plan of MESA Inc. (the "1991 STOCK OPTION PLAN"), such common
stock having been registered under the Securities Act of 1933 (the "Act")
pursuant to the MESA Registration Statement.

         In reaching the opinions set forth herein, we have examined and are
familiar with originals or copies, certified or otherwise identified to our
satisfaction, of such documents and records of the Company and MESA, and such
statutes, regulations and other instruments as we deemed necessary or advisable
for purposes of this opinion, including (i) the MESA Registration Statement,
(ii) the Post-Effective Amendment, (iii) the Restated Certificate of
Incorporation of the Company, as filed with the Secretary of State of the State
of Delaware, (iv) the Bylaws of the Company, and (v) certain minutes of
meetings of, and resolutions adopted by, the Board of Directors of the Company.

     We have assumed that (i) all information contained in all documents we
reviewed is true, correct and complete, (ii) all signatures on all documents we
reviewed are genuine, (iii) all documents submitted to us as originals are true
and complete, (iv) all documents submitted to us as copies are true and
complete copies of the originals thereof, and (v) all persons executing and
delivering the documents we examined were competent to execute and deliver such
documents. In addition, we have assumed that, upon exercise of the stock
options (the "OPTIONS") granted under the 1991 Stock Option Plan and assumed by
the Company pursuant to which shares of common


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Pioneer Natural Resources Company
September 8, 1997
Page 2

stock, par value $.01 per share, of the Company will be issued (the "SHARES"),
(i) the full consideration for each Share shall be paid to the Company and in
no event will such consideration be less than the par value for each Share, and
(ii) certificates evidencing the Shares will be properly executed and delivered
by the Company in accordance with the Delaware General Corporation Law (the
"DGCL").

     Based on the foregoing, and having due regard for the legal considerations
we deem relevant, we are of the opinion that the Shares, when issued by the
Company upon exercise of the Options, will be legally issued, fully paid and
non-assessable.

     This opinion is limited in all respects to the DGCL and the federal laws
of the United States of America. You should be aware that we are not admitted
to the practice of law in the State of Delaware.

     This opinion letter may be filed as an exhibit to the Post-Effective
Amendment. In giving this consent, we do not thereby admit that we come within
the category of persons whose consent is required under Section 7 of the Act or
the rules and regulations of the Commission promulgated thereunder.

                                           Very truly yours,

                                           /s/ Vinson & Elkins L.L.P.

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                                                                   EXHIBIT 23.1



                        CONSENT OF INDEPENDENT AUDITORS

The Board of Directors
Pioneer Natural Resources Company

We consent to the use of our reports incorporated herein by reference.


                                        KPMG PEAT MARWICK LLP

Midland, Texas
September 5, 1997

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                                                                    EXHIBIT 23.2



                   CONSENT OF INDEPENDENT PUBLIC ACCOUNTANTS

As independent public accountants, we hereby consent to the use of our reports
and to all references to our Firm included in or made a part of this
Registration Statement.


                                             ARTHUR ANDERSEN LLP

Dallas, Texas
September 5, 1997
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                                                                    EXHIBIT 23.3



                       CONSENT OF INDEPENDENT ACCOUNTANTS


We consent to the incorporation by reference in this Form S-8 registration
statement of Pioneer Natural Resources Company of our report dated July 26,
1996, on our audit of the financial statements of Greenhill Petroleum
Corporation as of June 30, 1996, and for the year ended.


                                        /s/ COOPERS & LYBRAND L.L.P.

                                            Coopers & Lybrand L.L.P.    


Houston, Texas
September 2, 1997

Data Provided by Refinitiv. Minimum 15 minutes delayed.