<PAGE>

                       THIRD AMENDMENT TO LOAN AGREEMENT
                       ---------------------------------

     THIS THIRD AMENDMENT TO LOAN AGREEMENT (herein called this "Amendment") is
made as of the 27th day of April, 2000 by and among Western Gas Resources, Inc.
("Borrower"), and Bank of America, N.A. ("Agent"), and the Lenders under the
Loan Agreement referred to below.

                             W I T N E S S E T H:

     WHEREAS, Borrower, Agent, and Lenders have entered into that certain Loan
Agreement dated as of April 29, 1999 (as amended, restated, or supplemented to
the date hereof, the "Original Agreement"), for the purposes and consideration
therein expressed, pursuant to which Lenders made and became obligated to make
loans to Borrower as therein provided;

     WHEREAS, Borrower, Agent, and Lenders desire to amend the Original
Agreement for the purposes described herein;

     NOW, THEREFORE, in consideration of the premises and the mutual covenants
and agreements contained herein and in the Original Agreement, in consideration
of the loans which may hereafter be made by Lenders to Borrower, and for other
good and valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto do hereby agree as follows:


                                  ARTICLE I.

                          Definitions and References
                          --------------------------

     1.I.    Defined Terms.  Unless the context otherwise requires or unless
             -------------
          otherwise expressly defined herein, the terms defined in the Original
          Agreement shall have the same meanings whenever used in this
          Amendment.  As used herein, the term "Loan Agreement" means the
          Original Agreement as amended by this Amendment.


                                  ARTICLE II.

                                   Amendment
                                   ---------

     2.I.    Definitions.  The definition of "Tranche A Maturity Date" in
             -----------
          Section 1.1 of the Loan Agreement is hereby amended in its entirety to
          read as follows:

          "'Tranche A Maturity Date' means April 26, 2001."
            -----------------------
<PAGE>

                                 ARTICLE III.

                                 Conditions of Effectiveness
                                 ---------------------------

       3.I.  Effective Date.  This Amendment shall become effective as of the
             --------------
date first above written when, and only when, Agent shall have received all of
                         -------------------
the following:

             (1)  This Amendment, duly authorized, executed and delivered by
     Borrower, Agent, and each Lender, and in form and substance satisfactory to
     Agent.

             (2)  A certificate of a duly authorized officer of Borrower dated
     the date of this Amendment certifying: (i) that all of the representations
     and warranties set forth in Article IV hereof are true and correct at and
     as of the time of such effectiveness; and (ii) as to such other corporate
     matters as Agent shall deem necessary.

             (c)  A written legal opinion of in-house counsel for Borrower,
     dated as of the date of this Amendment, addressed to Agent, to the effect
     that this Amendment has been duly authorized, executed and delivered by
     Borrower and that the Loan Agreement and each other Loan Document, as
     affected hereby, to which any Restricted Person is a party constitutes the
     legal, valid and binding obligation of each such Restricted Person,
     enforceable in accordance with their terms (subject, as to enforcement of
     remedies, to applicable bankruptcy, reorganization, insolvency and similar
     laws and to general principles of equity) and such other matters of Agent
     may require.

             (d)  Payment of all fees and expenses owing to Agent and Lenders in
     connection with the Loan Agreement and payment of fees and disbursements of
     Thompson & Knight L.L.P. relating to this Amendment and the Loan Agreement
     as provided in the Loan Agreement.

             (e)  Agent shall have additionally received such other documents as
     Agent may reasonably request.

                                  ARTICLE IV.

                        Representations and Warranties
                        ------------------------------

       4.I.  Representations and Warranties of Borrower.  In order to induce
             ------------------------------------------
each Lender to enter into this Amendment, Borrower represents and warrants to
each Lender that:

             (1)  The representations and warranties contained in Article V of
     the Original Agreement are true and correct at and as of the time of the
     effectiveness hereof (except as such representations and warranties have
     been modified by the transactions contemplated herein).

             (2)  Borrower is duly authorized to execute and deliver this
     Amendment and Borrower is and will continue to be duly authorized to borrow
     monies and to perform its

                                       2
<PAGE>

     obligations under the Loan Agreement. Borrower has duly taken all corporate
     action necessary to authorize the execution and delivery of this Amendment.

             (3)  The execution and delivery by Borrower of this Amendment, the
     performance of its obligations hereunder and the consummation of the
     transactions contemplated hereby do not and will not conflict with any
     provision of law, statute, rule or regulation or of the certificate of
     incorporation and bylaws of Borrower or of any material agreement,
     judgment, license, order or permit applicable to or binding upon Borrower
     or result in the creation of any lien, charge or encumbrance upon any
     assets or properties of Borrower.  Except for those which have been
     obtained, no consent, approval, authorization or order of any court or
     governmental authority or third party is required in connection with the
     execution and delivery by Borrower of this Amendment.

             (4)  When duly executed and delivered, this Amendment, the Loan
     Agreement, and each other Loan Document, as affected hereby, will be a
     legal and binding obligation of each Restricted Person that is a party
     hereto and thereto enforceable against such Restricted Person in accordance
     with its terms, except as limited by bankruptcy, insolvency or similar laws
     of general application relating to the enforcement of creditors' rights and
     by equitable principles of general application.

             (5)  The audited Consolidated financial statements of Borrower
     dated as of December 31, 1999 fairly present the Consolidated financial
     position at such date of Borrower and the Consolidated statement of
     operations and the changes in Consolidated financial position for the
     periods ending on such date for Borrower. Copies of such financial
     statements have heretofore been delivered to Agent. Since December 31,
     1999, no material adverse change has occurred in the financial condition or
     business or in the Consolidated financial condition or business of
     Borrower.

                                  ARTICLE V.

                                 Miscellaneous
                                 -------------

       5.I.  Ratification of Agreements.  The Original Agreement as hereby
             --------------------------
amended is hereby ratified and confirmed in all respects.  Any reference to the
Loan Agreement in any Loan Document shall be deemed to be a reference to the
Original Agreement as hereby amended.  The Loan Documents, as they may be
amended or affected by this Amendment, are hereby ratified and confirmed in all
respects.  The execution, delivery and effectiveness of this Amendment shall
not, except as expressly provided herein, operate as a waiver of any right,
power or remedy of Lenders under the Loan Agreement, the Notes, or any other
Loan Document nor constitute a waiver of any provision of the Loan Agreement,
the Notes, or any other Loan Document.

       5.II.  Survival of Agreements.  All representations, warranties,
              ----------------------
covenants and agreements of Borrower herein shall survive the execution and
delivery of this Amendment and the performance hereof, including without
limitation the making or granting of the Loans, and shall further survive until
all of the Obligations are paid in full.  All statements and agreements
contained in any certificate or instrument delivered by Borrower hereunder or
under the Loan

                                       3
<PAGE>

Agreement to any Lender shall be deemed to constitute representations and
warranties by, and/or agreements and covenants of, Borrower under this Amendment
and under the Loan Agreement.

       5.III.  Loan Documents.  This Amendment is a Loan Document, and all
               --------------
provisions in the Loan Agreement pertaining to Loan Documents apply hereto.

       5.IV.  Governing Law.  This Amendment shall be governed by and construed
              -------------
in accordance the laws of the State of Texas and any applicable laws of the
United States of America in all respects, including construction, validity and
performance.

       5.V.  Counterparts.  This Amendment may be separately executed in
             ------------
counterparts and by the different parties hereto in separate counterparts, each
of which when so executed shall be deemed to constitute one and the same
Amendment.

     THIS AMENDMENT AND THE OTHER LOAN DOCUMENTS REPRESENT THE FINAL AGREEMENT
BETWEEN THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR,
CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES.  THERE ARE NO
UNWRITTEN ORAL AGREEMENTS OF THE PARTIES.

                                       4
<PAGE>

     IN WITNESS WHEREOF, this Amendment is executed as of the date first above
written.


                              WESTERN GAS RESOURCES, INC.



                              By:_____________________________________
                                 Name:
                                 Title:

                                       5