<PAGE>
                                                                    EXHIBIT 12.5
================================================================================



                                LOAN AGREEMENT



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



                          WESTERN GAS RESOURCES, INC.

                                   Borrower


                                      and


                               NATIONSBANK, N.A.

                                    as Bank

                                      and

                     NationsBanc Montgomery Securities LLC

                               as Lead Arranger



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


                                  $37,000,000

                               February 17, 1999


================================================================================
<PAGE>
 
                               NATIONSBANK, N.A.
                                901 Main Street
                              Dallas, Texas 75202

                               February 17, 1999




Western Gas Resources, Inc.
1200 N. Pecos Street, Suite 230
Denver, Colorado 80234

Ladies and Gentlemen:

     This Agreement (this "Agreement") will serve to set forth the terms of
                           ---------
certain financing transactions by and between Western Gas Resources, Inc., a
Delaware corporation ("Borrower"), and NationsBank, N.A. ("Bank"). Capitalized
                       --------                            ----
terms used but not defined herein (i) shall have the meanings given them on
Annex I hereto or (ii) if not defined on Annex I hereto, shall have the meanings
given them in the Revolving Credit Agreement.

     1.   BRIDGE LOAN.
          -----------

     (a)  Subject to the terms and conditions hereof, Bank agrees to make a
single advance to Borrower in an amount not to exceed $37,000,000 (the "Bridge
                                                                        ------
Loan") on or before February 23, 1999. Borrower must give to Bank written notice
----
of the date it requests that the Bridge Loan be made, no later than 10:00 am,
Dallas time, on such day.

     (b)  The obligation of Borrower to repay to Bank the Bridge Loan, together
with interest accruing in connection therewith, shall be evidenced by a single
promissory note made by Borrower payable to the order of Bank (the "Bridge
Note") in the form of Exhibit A with appropriate insertions. The amount of
principal owing on the Bridge Note at any given time shall be the amount of the
Bridge Loan minus all payments of principal theretofore received by Bank on the
Bridge Note. Interest on the Bridge Note shall accrue and be payable as provided
therein.

     (c)  Borrower shall use all funds from the Bridge Loan to repay in part
existing outstanding indebtedness owing under the Debt Securities. In no event
shall the proceeds of the Bridge Loan be used directly or indirectly for the
purpose, whether immediate, incidental or ultimate, of purchasing, acquiring or
carrying any "margin stock" or any "margin securities" (as such terms are
defined respectively in Regulation U and Regulation G promulgated by the Board
of Governors of the Federal Reserve System) or to extend credit to others
directly or indirectly
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Western Gas Resources, Inc.
February 17, 1999
Page 2


for the purpose of purchasing or carrying any such margin stock or margin
securities. Borrower represents and warrants to Bank that it is not engaged
principally, or as one of its important activities, in the business- of
extending credit to others for the purpose of purchasing or carrying such margin
stock or margin securities.

     (d)  Borrower will make each payment which it owes hereunder or under the
Bridge No e not later than noon, Dallas time, in lawful money of the United
States of America and in immediately available funds. Any payment received by
Bank after such time will be deemed to have been made oil the next following
Business Day. Should any such payment become due and payable on a day other than
a Business Day, the maturity of such payment shall be extended to the next
succeeding Business Day, and, in the case of a payment of principal or past due
interest, interest shall accrue and be payable thereon for the period of such
extension as provided in the Agreement Document under which such payment is due.
Each payment under a Bridge Loan Document shall be payable at the place provided
therein and, if no specific place of payment is provided, shall be payable at
the place of payment of the Bridge Note. When Bank collects or receives money on
account of the Bridge Loan Obligations, Bank shall apply all such money so
distributed, as follows:

          (i)   for the prepayment of amounts owing under the Bridge Loan
     Documents;

          (ii)  then for the prepayment of principal on the Bridge Note,
     together with accrued and unpaid interest on the principal so prepaid; and

          (iii) last, for the payment or prepayment of any other Bridge Loan
     Obligations.

All payments applied to principal or interest on the Bridge Note shall be
applied first to any interest then due and payable, then to principal then due
and payable, and last to any prepayment of principal and interest in compliance
with this Agreement.

     2.   RATE ELECTIONS. Borrower may from time to time designate all or any
          --------------
portion of the Bridge Loan as a "Eurodollar Portion". Borrower may make no such
election during the continuance of a Default, and Borrower may make such an
election with respect to already existing Eurodollar Portions only if such
election will take effect at or after the termination of the Interest Period
applicable to such already existing Fixed Rate Portions. Each election by
Borrower of a Eurodollar Portion shall:
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Western Gas Resources, Inc.
February 17, 1999
Page 3


     (a)  Be made by written notice to Bank or by telephonic notice to Bank
promptly confirmed in writing, in the form and substance of the "Rate Election"
attached hereto as Exhibit B, duly completed and signed by an authorized officer
of Borrower.

     (b)  Specify the aggregate amount of the Bridge Loan which Borrower desires
to designate as such Eurodollar Portion, the first day of the Interest Period
which is to apply thereto, and the length of such Interest Period; and

     (c)  Be received by Bank not later than 10:00 a.m., Dallas time, on the
second Business Day preceding the first day of the specified Interest Period.

Each such election (herein called a "Rate Election") shall be irrevocable.
                                     -------------
Borrower may make no Rate Election which does not specify an Interest Period
complying with the definition of "Interest Period" in Annex L and the amount of
each Eurodollar Portion elected in any Rate Election must be $ 10,000,000 or a
higher integral multiple of $1,000,000. Upon the termination of each Interest
Period the portion of the Bridge Loan theretofore constituting the related
Eurodollar Portion shall, unless the subject of a new Rate Election then taking
effect, automatically become a part of the Base Rate Portion of the Bridge Loan
and become subject to all provisions of the Bridge Loan Documents governing such
Base Rate Portion. Borrower shall have no more than three Eurodollar Portions in
effect at any time.

     3.   OPTIONAL PREPAYMENTS. Borrower may, upon concurrent notice to Bank,
          --------------------
from time to time and without premium or penalty prepay the Bridge Loan, in
whole or in part, so long as each partial prepayment of principal is in an
amount greater than or equal to $1,000,000, provided that any prepayment of a
Eurodollar Portion shall be accompanied by all amounts due under Section 7. Each
prepayment of principal under this section shall be accompanied by all interest
then accrued and unpaid on the principal so prepaid. Any principal or interest
prepaid pursuant to this section shall be in addition to, and not in lieu of,
all payments otherwise required to be paid under the Bridge Loan Documents at
the time of such prepayment.

     4.   MANDATORY PREPAYMENTS. If (i) Borrower shall sell, exchange, lease,
          ---------------------
transfer, or otherwise dispose of any material part of, or material interest in
any of its assets having a sales price in excess of $1,000,000 or (ii) Borrower
shall issue any public debt or equity instrument, Borrower shall prepay the
principal of the Bridge Loan within two (2) Business Days of such transaction in
an amount at least equal to (a) ninety percent (90%) of the Fair Market Value of
the portion or interest in such assets sold, exchanged, leased, transferred or
otherwise disposed of as of the date of such transaction or (b) one hundred
percent (100%) of the proceeds of such issuance of such debt or equity
instrument (net of Borrower's reasonable costs of sale or
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 4


issuance). Each prepayment of principal under this section shall be accompanied
by all interest then accrued and unpaid on the principal so prepaid. Any
principal prepaid pursuant to this section shall be in addition to, and not in
lieu of, all payments otherwise required to be paid under the Bridge Loan
Documents at the time of such prepayment.

     5.   INCREASED COST OF EURODOLLAR PORTIONS. If any applicable domestic or
          -------------------------------------
foreign law, treaty, rule or regulation (whether now in effect or hereinafter
enacted or promulgated, including Regulation D) or any interpretation or
administration thereof by any governmental authority charged with the
interpretation or administration thereof (whether or not having the force of
law):

     (a)  shall change the basis of taxation of payments to Bank of any
principal, interest, or other amounts attributable to any Eurodollar Portion or
otherwise due under this Agreement in respect of any Eurodollar Portion (other
than taxes imposed on the overall net income of Bank or any lending office of
Bank by any jurisdiction in which Bank or any such lending office is located);
or

     (b)  shall change, impose, modify, apply or deem applicable any insurance
fees or premiums or any reserve, special deposit or similar requirements in
respect of any Eurodollar Portion (excluding those for which there is full
compensation pursuant to adjustments made in the definition of Adjusted
Eurodollar Rate), or against assets of, deposits with or for the account of, or
credit extended by, Bank; or

     (c)  shall impose on Bank or the interbank eurocurrency deposit market any
other condition affecting any Eurodollar Portion, the result of which is to
increase the cost to Bank of funding or maintaining any Eurodollar Portion or to
reduce the amount of any sum receivable by Bank in respect of any Eurodollar
Portion by an amount deemed by Bank to be material,

then Bank shall promptly notify Borrower in writing of the happening of such
event and (1) Borrower shall upon demand pay to Bank such additional amount or
amounts as will compensate Bank for such event and (2) Borrower may elect, by
giving to Bank not less than three Business Days' notice, to convert all (but
not less than all) of any such Eurodollar Portion into a part of the Base Rate
Portion.

     6.   CHANGE OF LAW. If any change in applicable laws, treaties, rules or
          -------------
regulations or in the interpretation or administration thereof or in any
jurisdiction whatsoever, domestic or foreign, shall make it unlawful or
impracticable for Bank to fund or maintain Eurodollar Portions, or shall
materially restrict the authority of Bank to purchase, sell or take offshore
deposits of dollars (i.e., "eurodollars"), then Borrower's right to elect
                     ---
Eurodollar Portions shall be suspended to the
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 5


extent and for the duration of such illegality, impracticability or restriction
and all Eurodollar Portions (or portions thereof) which are then outstanding or
are then the subject of any Rate Election and which cannot lawfully or
practicably be maintained or funded shall immediately become or remain part of
the Base Rate Portion of the Bridge Loan. Borrower agrees to indemnify Bank and
hold it harmless against all costs, expenses, claims, penalties, liabilities and
damages which actually result from any such change in law, treaty, rule,
regulation, interpretation or administration.

     7.   FUNDING LOSSES. Upon the request of Bank, Borrower shall pay to Bank
          --------------
such amount or amounts as shall be sufficient (in the reasonable opinion of
Bank) to compensate Bank for any loss or expense incurred or sustained by Bank
(including without limitation any loss or expense incurred by reason of the
liquidation or reemployment of deposits or other funds acquired by Bank to fund
or maintain Eurodollar Portions hereunder), as a result of (a) any payment or
prepayment (whether authorized or required hereunder or otherwise) of all or a
portion of a Eurodollar Portion on a day other than the day on which the
applicable Interest Period ends, (b) any payment or prepayment, whether required
hereunder or otherwise, of the Bridge Loan made after the delivery, but before
the effective date, of a Rate Election, if such payment or prepayment prevents
such Rate Election from becoming fully effective, (c) the failure of" any Rate
Election to become effective, or (d) any conversion (whether authorized or
required hereunder or otherwise) of all or any portion of any Eurodollar Portion
into a Base Rate Portion or into a different Eurodollar Portion on a day other
than the day on which the applicable Interest Period ends.

     8.   REIMBURSABLE TAXES; CAPITAL ADEQUACY. Borrower covenants and agrees
          ------------------------------------
that, whether or not any Eurodollar Portion is ever elected:

     (a)  Borrower will reimburse Bank on demand, on an after-tax basis, for
all present and future income, stamp and other taxes, levies, costs and charges
whatsoever actually paid by Bank (or required to be withheld and paid on account
of Bank) in respect of any Eurodollar Portions, excluding, however, any thereof
imposed on or measured by the overall net income of Bank or any lending office
of Bank by any jurisdiction in which Bank or any such lending office is located
(all such non-excluded taxes, levies, costs and charges being collectively
called "Reimbursable Taxes" in this section), and Borrower will pay directly to
        ------------------
the appropriate authority any Reimbursable Taxes which Borrower is required to
withhold and pay. Promptly after the date on which payment of any such
Reimbursable Tax to be paid directly by Borrower is due or claimed to be due,
Borrower will, at the request of Bank, furnish to Bank evidence in form and
substance satisfactory to Bank that Borrower has met its obligation under this
section.
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 6


     (b)  Borrower will indemnify Bank against any loss, liability, claim or
expense, including interest, penalties and legal fees, that Bank may incur at
any time arising out of or in connection with the failure of Borrower to make
any reimbursement required under subsection (a) above or to make any payment,
when due or claimed to be due, of Reimbursable Taxes to be withheld and paid
directly by Borrower.

     (c)  All payments on account of the principal of, and interest on, the
Bridge Loan and the Bridge Note, and all other amounts payable by Borrower to
Bank hereunder shall be made free and clear of and without reduction by reason
of any Reimbursable Taxes, all of which will be for the account of Borrower and
reimbursed or paid by Borrower.

     (d)  If Borrower is ever required to reimburse or pay any Reimbursable Tax
with respect to any Eurodollar Portion, Borrower may elect, by giving to Bank
not less than three Business Days' notice, to convert all (but not less than
all) of any such Eurodollar Portion into apart of the Base Rate Portion of the
Bridge Loan, but such election shall not diminish Borrower's obligation to pay
all Reimbursable Taxes.

     (e)  If at any time after the date hereof, and from time to time, Bank
determines that the adoption or modification of any applicable law, rule or
regulation regarding taxation, Bank's required levels of reserves, deposits,
insurance or capital (including any allocation of capital requirements or
conditions), or similar requirements, or any interpretation or administration
thereof by any governmental authority, central bank or comparable agency charged
with the interpretation, administration or compliance of Bank with any of such
requirements, has or would have the effect of (a) increasing Bank's costs
relating to the Bridge Loan Obligations, or (b) reducing the yield or rate of
return of Bank on the Bridge Loan Obligations, to a level below that which Bank
could have achieved but for such adoption, modification, interpretation or
application, then Borrower shall, within 30 Business Days after any request by
Bank, pay to Bank such additional amounts as (in Bank's sole judgment, after
reasonable computation) will compensate Bank for such increase in costs or
reduction in yield or rate of return. No failure by Bank to immediately demand
payment of any additional amounts payable hereunder shall constitute a waiver of
Bank's right to demand payment of such amounts at any subsequent time. Nothing
herein contained shall be construed or so operate as to require Borrower to pay
any interest, fees, costs or charges not permitted by Section 9.

     9.   COMPENSATION PROCEDURE. If Bank notifies Borrower of the incurrence of
          ----------------------
additional costs under Sections 5 through 8, Bank shall in such notice to
Borrower set forth in reasonable detail the basis and amount of its request for
compensation. Determinations and allocations by Bank for purposes of Sections 5
through 8 of the effect of any change in
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 7


applicable laws, treaties, rules or regulations or in the interpretation or
administration thereof, any losses or expenses incurred by reason of the
liquidation or reemployment of deposits or other funds, any taxes, levies, costs
and charges imposed, or the effect of capital maintained on its costs or rate of
return of maintaining the Bridge Loan, or on amounts receivable by it in respect
of the Bridge Loan and of the amounts required to compensate Bank under Sections
5 through 8, shall be conclusive and binding for all purposes, provided that
such determinations and allocations are made on a reasonable basis and there
shall be no demand for duplicate payments of the same additional cost. Any
request for compensation under this Section 9 shall be paid by Borrower within
thirty (30) Business Days of the receipt by Borrower of the notice described in
this Section 9.

     10.  CONDITIONS PRECEDENT. Bank has no obligation to make the Bridge Loan
          --------------------
unless all of the following conditions shall have been satisfied:

     (a)  Bank has received all of the following, at Bank's offices, duly
executed and delivered and in form, substance and date satisfactory to Bank:

          (i)    This Agreement.

          (ii)   The Bridge Note.

          (iii)  An absolute and unconditional guaranty of the Bridge Loan
     Obligations executed by each Subsidiary of Borrower in a form acceptable to
     Bank.

          (iv)   An officer's certificate of Borrower dated as of the date of
     the making of the Bridge Loan, which shall contain the names and signatures
     of the officers of Borrower duly authorized to execute the Bridge Loan
     Documents.

          (v)    Such supporting documents as Bank may reasonably request.

     (b)  All representations and warranties made by Borrower in the Revolving
Credit Agreement shall be true on and as of the date of the Bridge Loan (except
to the extent that the facts upon which such representations are based have been
changed by the transactions therein contemplated) as if such representations and
warranties had been made as of the date of the Bridge Loan.

     (c)  No Default shall exist at the date of the Bridge Loan.
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 8


     (d)  Borrower shall have performed and complied with all agreements and
conditions required in the Bridge Loan Documents to be performed or complied
with by it on or prior to the date of the Bridge Loan.

     (e)  All legal matters relating to the Bridge Loan Documents and the
consummation of the transactions contemplated thereby shall be satisfactory to
Thompson & Knight, a Professional Corporation, counsel to Bank.

     (f)  Borrower shall have paid to Bank a commitment fee in the amount of
$185,000 in immediately available funds.

     (g)  Majority Lenders shall have consented in writing to the execution and
delivery by Borrower and Bank of this Agreement.

     11.  REPRESENTATIONS AND WARRANTIES. Borrower hereby represents and
          ------------------------------
warrants to Bank that each of the representations and warranties contained in
Section 5.1 of the Revolving Credit Agreement are true and correct as of the
date hereof and as of the date the Bridge Loan is made (except to the extent
that the facts upon which such representations are based have been changed by
the transactions therein contemplated).

     12.  AFFIRMATIVE COVENANTS. Until (i) the Bridge Note is fully paid and
          ---------------------
satisfied, and (ii) this Agreement is terminated, Borrower agrees and covenants
that it will, unless Bark shall otherwise consent in writing, comply with each
provision of Section 6.1 and Section 6.2 of the Revolving Credit Agreement.

     13.  BANK ACCOUNTS; OFFSET. To secure the repayment of the Bridge Loan,
          ---------------------
Borrower hereby grants to Bank a security interest, a lien, and a right of
offset, each of which shall be upon and against all right, title, and interest
of Borrower in (a) any and all moneys, securities or other property (and the
proceeds therefrom) of Borrower now or hereafter held or received by or in
transit to Bank from or for the account of Borrower, whether for safekeeping,
custody, pledge, transmission, collection or otherwise, (b) any and all deposits
(general or special, time or demand, provisional or final) of Borrower with
Bank, and (c) any other credits and claims of Borrower at any time existing
against Bank, including claims under certificates of deposit. Upon the
occurrence of any Default, Bank is hereby authorized to foreclose upon, offset,
appropriate, and apply, at any time and from time to time, without notice to
Borrower or any Guarantor, any and all items hereinabove referred to against the
Bridge Loan Obligations (whether or not such Bridge Loan Obligations are then
due and payable).
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Western Gas Resources, Inc.
February 17, 1999
Page 9


     14.  GUARANTIES OF SUBSIDIARIES. Each Subsidiary of Borrower which (i) is
          --------------------------
now existing shall on date hereof or (ii) is created, acquired or comes into
existence after the date hereof shall promptly execute and deliver to Bank an
absolute and unconditional guaranty of the timely repayment of the Bridge Loan
Obligations, which guaranty shall be satisfactory to Bank in form and substance.
Borrower and each Guarantor will cause each such Related Person to deliver to
Bank, simultaneously with its delivery of such a guaranty, written evidence
satisfactory to Bank and its counsel that such Related Person has taken all
corporate or partnership action necessary to duly approve and authorize its
execution, delivery and performance of such guaranty and any other documents
which it is required to execute.

     15.  FURTHER ASSURANCES. Borrower hereby- agrees to, and to cause each
          ------------------
Related Person to, execute and deliver any and all documents, instruments,
guaranties, agreements, certificates, and other writings of any kind and
character which Bank determines in its sole and absolute discretion are
necessary or appropriate to effectuate the intent of the Bridge Loan Documents.

     16.  EVENTS OF DEFAULT. Each of the following shall constitute an "Event of
          -----------------
Default" under this Agreement:

     (a)  Any Related Person fails to pay any Bridge Loan Obligations when due
and payable, whether at. a date for the payment of a fixed installment or
contingent or other payment to Bank or as a result of acceleration or otherwise;

     (b)  Any Related Person fails (other than as referred to in subsections
(a) and (b) above) to duly observe, perform or comply with any covenant,
agreement, condition or provision of any Bridge Loan Document;

     (c)  An "Event of Default" occurs under the Revolving Credit Agreement.

     17.  REMEDIES. Upon the occurrence of any one or more of the foregoing
          --------
Events of Default, the entire unpaid balance of principal of the Bridge Note,
together with all accrued but unpaid interest thereon, and all other
indebtedness owing to Bank by Borrower at such time shall, at the option of
Bank, become immediately due and payable without further notice, demand,
presentation, notice of dishonor, notice of intent to accelerate, notice of
acceleration, protest or notice of protest of any kind, all of which are
expressly waived by Borrower; provided, however concurrently and automatically
                              --------  -------
with the occurrence of an "Event of Default" under Section 8.1 (h)(i), (ii), or
(iii) of the Revolving Credit Agreement, the Bridge Note and all other
indebtedness owing to Bank by Borrower under the Bridge Loan Documents at such
time shall, without any action by Bank, become due and payable, without further
notice, demand,
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 10


presentation, notice of dishonor, notice of acceleration, notice of intent to
accelerate, protest or notice of protest of any kind, all of which are expressly
waived by Borrower. All rights and remedies of Bank set forth in this Agreement
and in any of the other Bridge Loan Documents may also be exercised by Bank, at
its option to be exercised in its sole discretion, upon the occurrence of an
Event of Default.

     18.  RIGHTS; CUMULATIVE. All rights of Bank under the terms of this
          ------------------
Agreement shall be cumulative of, and in addition to, the rights of Bank under
any and all other agreements between Borrower and Bank (including, but not
limited to, the other Bridge Loan Documents), and not in substitution or
diminution of any rights now or hereafter held by Bank under the terms of any
other agreement.

     19.  WAIVER AND AGREEMENT. Neither the failure nor any delay on the part
          --------------------
of Bank to exercise any right, power or privilege herein or under any of the
other Bridge Loan Documents shall operate as a waiver thereof, nor shall any
single or partial exercise of such right, power or privilege preclude any other
or further exercise thereof or the exercise of any other right, power or
privilege. No waiver of any provision in this Agreement or in any of the other
Bridge Loan Documents and no departure by Borrower therefrom shall be effective
unless the same shall be in writing and signed by Bank, and then shall be
effective only in the specific instance and for the purpose for which given and
to the extent specified in such writing. No modification or amendment to this
Agreement or to any of the other Bridge Loan Documents shall be valid or
effective unless the same is signed by the Party against whom it is sought to be
enforced.

     20.  BENEFITS. This Agreement shall be binding upon and inure to the
          --------
benefit of Bank and Borrower, and their respective successors and assigns,
provided, however, that Borrower may not, without the prior written consent of
Bank, assign any rights, powers, duties or obligations under this Agreement or
any of the other Bridge Loan Documents.

     21.  NOTICES. All notices, requests, demands or other communications
          -------
required or permitted to be given pursuant to this Agreement shall be in writing
and given by (i) facsimile, (ii) personal delivery, (iii) expedited delivery
service with proof of delivery, or (iv) United States mail, postage prepaid,
registered or certified mail, return receipt requested, sent to the intended
addressee at the address set forth on the signature page hereof and shall be
deemed to have been received either, in the case of facsimile or personal
delivery, as of the time of facsimile or personal delivery, in the case of
expedited delivery service, as of the date of first attempted delivery at the
address and in the manner provided herein, or in the case of mail, upon deposit
in a depository receptacle under the care and custody of the United States
Postal Service. Either party shall have the right to change its address for
notice hereunder to any other location within
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 11


the continental United States by notice to the other party of such new address
at least thirty (30) days prior to the effective date of such new address. All
notices required to be given by Bank to any Related Person other than Borrower
pursuant to any Bridge Loan Document shall be given to such Related Person at
the address of Borrower on the signature page hereof in accordance with this
Section 21.

     22.  LIMITATION ON INTEREST. Bank and Borrower intend to contract in
          ----------------------
strict compliance with applicable usury law from time to time in effect. In
furtherance thereof Bank and Borrower stipulate and agree that none of the terms
and provisions contained in the Agreement shall ever be construed to create a
contract to pay, for the use, forbearance or detention of money, interest in
excess of the maximum amount of interest permitted to be charged by applicable
Law from time to time in effect. Neither Borrower nor any present or future
guarantors, endorsers, or other Persons hereafter becoming liable for payment of
any Obligation shall ever be liable for unearned interest thereon or shall ever
be required to pay interest thereon in excess of the maximum amount that may be
lawfully charged under applicable Law from time to time in effect, and the
provisions of this section shall control over all other provisions of the
Agreement which may be in conflict or apparent conflict herewith. Bank expressly
disavows any intention to charge or collect excessive unearned interest or
finance charges in the event the maturity of any Obligation is accelerated. If
(a) the maturity of any Obligation is accelerated for any reason, (b) any
Obligation is prepaid and as a result any amounts held to constitute interest
are determined to be in excess of the legal maximum, or (c) Bank or any other
holder of any or all of the Bridge Loan Obligations shall otherwise collect
moneys which are determined to constitute interest which would otherwise
increase the interest on any or all of the Bridge Loan Obligations to an amount
in excess of that permitted to be charged by applicable Law then in effect, then
all sums determined to constitute interest in excess of such legal limit shall,
without penalty, be promptly applied to reduce the then outstanding principal of
the related Bridge Loan Obligations or, at Bank's or such holder's option,
promptly returned to Borrower or the other payor thereof upon such
determination. In determining whether or not the interest paid or payable, under
any specific circumstance, exceeds the maximum amount permitted under applicable
Law, Bank and Borrower (and any other payors thereof) shall to the greatest
extent permitted under applicable Law, (i) characterize any non-principal
payment as an expense, fee or premium rather than as interest, (ii) exclude
voluntary prepayments and the effects thereof, and (iii) amortize, prorate,
allocate, and spread the total amount of interest throughout the entire
contemplated ' term of the instruments evidencing the Bridge Loan Obligations in
accordance with the amounts outstanding from time to time thereunder and the
maximum legal rate of interest from time to time in effect under applicable Law
in order to lawfully charge the maximum amount of interest permitted under
applicable Law. In the event applicable Law provides for an interest ceiling
under Chapter 303 of the Texas Finance Code (the "Texas Finance Code") as
amended, for that day, the ceiling
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Western Gas Resources, Inc.
February 17, 1999
Page 12


shall be the "weekly ceiling" as defined in the Texas Finance Code, provided
that if any applicable Law permits greater interest, the Law permitting the
greatest interest shall apply. As used in this section the term "applicable Law"
means the laws of the State of Texas or the laws of the United States of
America, whichever laws allow the greater interest, as such laws now exist or
may be changed or amended or come into effect in the future.

     23.  CONSTRUCTION. This Agreement and the other Bridge Loan Documents have
          ------------
been executed and delivered in the State of Texas, shall be governed by and
construed in accordance with the laws of the State of Texas.

     24.  INVALID PROVISIONS. If any provision of this Agreement or any of the
          ------------------
other Bridge Loan Documents is held to be illegal, invalid or unenforceable
under present or future laws, such provision shall be fully severable and the
remaining provisions of this Agreement or any of the other Bridge Loan Documents
shall remain in full force and effect and shall not be affected by the illegal,
invalid or unenforceable provision or by its severance.

     25.  EXPENSES. Borrower agrees to pay on demand all reasonable costs and
          --------
expenses of Bank in connection with the syndication, preparation, execution,
delivery, administration, Modification, and amendment of this Agreement, the
other Bridge Loan Documents, and the other documents to be delivered hereunder,
including, without limitation, the reasonable fees and expenses of counsel for
Bank (including the cost of internal counsel) with respect thereto and with
respect to advising Bank as to its rights and responsibilities under the Bridge
Loan, Documents. Borrower further agrees to pay on demand all costs and expenses
of Bank, if any (including, without limitation, reasonable attorneys' fees and
expenses and the cost of internal counsel), in connection with the enforcement
(whether through negotiations, legal proceedings, or otherwise) of the Bridge
Loan Documents and the other documents to be delivered hereunder. The agreements
contained in this Section shall survive payment in full of the Bridge Loan and
all other amounts payable under this Agreement.

     26.  PARTICIPATION OF THE BRIDGE LOAN. Borrower agrees that Bank may, at
          --------------------------------
its option, sell interests in the Bridge Loan and its rights under this
Agreement to a financial institution or institutions and, in connection with
each such sale, Bank may disclose any financial and other information available
to Bank concerning Borrower to each perspective purchaser.

     27.  ENTIRE AGREEMENT. THIS AGREEMENT, TOGETHER WITH THE OTHER BRIDGE LOAN
          ----------------
DOCUMENTS, REPRESENTS THE FINAL AGREEMENT BETWEEN THE PARTIES AND MAY NOT BE
CONTRADICTED BY EVIDENCE OF
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 13


PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE ARE
NO UNWRITTEN ORAL AGREEMENTS OF THE PARTIES.

     28.  CONFLICTS. In the event any term or provision hereof is inconsistent
          ---------
with or conflicts with any provision of the other Bridge Loan Documents, the
terms and provisions contained in this Agreement shall be controlling.

     29.  COUNTERPARTS. This Agreement may be separately executed in any number
          ------------
of counterparts, each of which shall be an original, but all of which, taken
together, shall be deemed to constitute one and the same instrument. This
Agreement may be validly executed and delivered by facsimile or other electronic
transmission.

     30.  INDEMNITY. Borrower agrees to indemnify Bank, upon demand, from and
          ---------
against any and all liabilities, obligations, claims, losses, damages,
penalties, actions, judgments, suits, costs, expenses or disbursements
(including reasonable fees of attorneys, accountants, experts and advisors) of
any kind or nature whatsoever (in this section collectively called "liabilities
and costs") which to any extent (in whole or in part) may be imposed on,
incurred by, or asserted against Bank growing out of, resulting from or in any
other way associated with any of the collateral, the Bridge Loan Documents and
the transactions and events (including without limitation the enforcement or
defense thereof) at any time associated therewith or contemplated therein
(including any violation or noncompliance with any environmental laws or any
liabilities as duties with respect to hazardous materials found in or released
into the environment). THE FOREGOING INDEMNIFICATION SHALL APPLY WHETHER OR NOT
SUCH LIABILITIES AND COSTS ARE IN ANY WAY OR TO ANY EXTENT ARISE, IN WHOLE OR IN
PART, UNDER ANY CLAIM OR THEORY OF STRICT LIABILITY, OR ARE CAUSED, IN WHOLE OR
IN PART, BY ANY NEGLIGENT ACT OR OMISSION OF ANY KIND BY BANK, provided only
that Bank shall be not entitled under this section to receive indemnification
for that portion, if any, of any liabilities and costs which is proximately
caused by its own individual gross negligence or willful misconduct. As used in
this section the term "Bank" shall refer not only to NationsBank, N.A. but also
to each director, officer, Bank, attorney, employee, representative and
affiliate of such person.

     31.  SURVIVAL OF AGREEMENTS; CUMULATIVE NATURE. All of the Related
          -----------------------------------------
Persons' various representations, warranties, covenants and agreements in the
Bridge Loan Documents shall survive the execution and delivery of this Agreement
and the other Bridge Loan Documents and the performance hereof and thereof,
including the making or granting of the Bridge Loan and the delivery of the
Bridge Note and the other Bridge Loan Documents, and shall further survive until
all of the Bridge Loan Obligations are paid in full and all of Bank's
obligations to Borrower
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 14


pursuant to this Agreement are terminated. All statements and agreements
contained in any certificate or other instrument delivered by any Related Person
to Bank under any Bridge Loan Document shall be deemed representations and
warranties by Borrower or agreements and -covenants of Borrower under this
Agreement. The representations, warranties, indemnities, and covenants made by
the Related Persons in the Bridge Loan Documents, and the rights, powers, and
privileges granted to Bank in the Bridge Loan Documents, are cumulative, and,
except for expressly specified waivers and consents, no Bridge Loan Document
shall be construed in the context of another to diminish, nullify, or otherwise
reduce the benefit to Bank of any such representation, warranty, indemnity,
covenant, right, power or privilege. In particular and without limitation, no
exception set out in this Agreement to any representation, warranty, indemnity,
or covenant herein contained shall apply to any similar representation,
warranty, indemnity, or covenant contained in any other Bridge Loan Document,
and each such similar representation, warranty, indemnity, or covenant shall be
subject only to those exceptions which are expressly made applicable to it by
the terms of the various Bridge Loan Documents.

     32.  LIMITED SURVIVAL UPON TERMINATION. Upon the payment in full of the
          ---------------------------------
Bridge Loan Obligations and the termination of this Agreement and
notwithstanding anything herein to the contrary, any waivers or admissions made
by any Related Person in any Bridge Loan Document, and any obligations which any
Person may have to indemnify or compensate Bank shall survive any termination of
this Agreement or any other Bridge Loan Document. At the request and expense of
Borrower, Bank shall prepare and execute all necessary instruments to reflect
and effect such termination of the Bridge Loan Documents.

     33.  WAIVERS AND AMENDMENTS; ACKNOWLEDGMENTS.
          ---------------------------------------

     (a)  No failure or delay (whether by course of conduct or otherwise) by
Bank in exercising any right, power or remedy which Bank may have under any of
the Bridge Loan Documents shall operate as a waiver thereof or of any other
right, power or remedy, nor shall any single or partial exercise by Bank of any
such right, power or remedy preclude any other or further exercise thereof or of
any other right, power or remedy. No failure or delay (whether by course of
conduct or otherwise) by Borrower in exercising any right, power or remedy which
Borrower may have under any of the Bridge Loan Documents shall operate as a
waiver thereof or of any other right, power or remedy, nor shall any single or
partial exercise by Borrower of any such right, power or remedy preclude any
other or further exercise thereof or of any other right, power or remedy. No
waiver of any provision of any Bridge Loan Document and no consent to any
departure therefrom shall ever be effective unless it is in writing and signed
as provided below in this section, and then such waiver or consent shall be
effective only in the specific instances and for the purposes for which given
and to the extent specified in such writing. No
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 15


notice to or demand on any Related Person shall in any case of itself entitle
any Related Person to any other or further notice or demand in similar or other
circumstances.

     (b)  No waiver or supplement to this Agreement or the other Bridge Loan
Documents shall be valid or effective against any party hereto unless the same
is in writing and signed by (i) if such party is Borrower, by Borrower, and (ii)
if such party is Bank, by Bank.

     (c)  Borrower hereby represents, warrants, acknowledges and admits that
(i) it has been advised by counsel in the negotiation, execution and delivery of
the Bridge Loan Documents to which it is a party, (ii) it has made an
independent decision to enter into this Agreement and the other Bridge Loan
Documents to which it is a party, without reliance on any representation,
warranty, covenant or undertaking by Bank, whether written, oral or implicit,
other than as expressly set out in this Agreement or in another Bridge Loan
Document delivered on or after the date hereof, (iii) there are no
representations, warranties, covenants, undertakings or agreements by Bank as to
the Bridge Loan Documents except as expressly set out in this Agreement or in
another Bridge Loan Document delivered on or after the date hereof, (iv) Bank
has no fiduciary obligation toward Borrower with respect to any Bridge Loan
Document or the transactions contemplated thereby, (v) the relationship pursuant
to the Bridge Loan Documents between Borrower, oil one hand, and Bank, on the
other hand, is and shall be solely that of debtor and creditor, respectively,
(vi) no partnership or joint venture exists with respect to the Bridge Loan
Documents between Borrower and Bank, (vii) should an Event of Default or Default
occur or exist Bank will determine in its sole discretion and for its own
reasons what remedies and actions it will or will not exercise or take at that
time, (viii) without limiting any of the foregoing, Borrower is not relying upon
any representation or covenant by Bank, or any representative thereof, and no
such representation or covenant has been made, that Bank will, at the time of an
Event of Default or Default, or at any other time, waive, negotiate, discuss, or
take or refrain from taking any action permitted under the Bridge Loan Documents
with respect to any such Event of Default or Default or any other provision of
the Bridge Loan Documents, and (ix) Bank has relied upon the truthfulness of the
acknowledgments in this section in deciding to execute and deliver this
Agreement and to make the Bridge Loan.

     THIS WRITTEN AGREEMENT AND THE OTHER BRIDGE 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.
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 16


     THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

     34.  WAIVER OF JURY TRIAL, PUNITIVE DAMAGES, ETC. EACH OF BORROWER AND
          -------------------------------------------
BANK HEREBY KNOWINGLY, VOLUNTARILY, INTENTIONALLY, AND IRREVOCABLY (A) WAIVES,
TO THE MAXIMUM EXTENT NOT PROHIBITED BY LAW, ANY RIGHT IT MAY HAVE TO A TRIAL BY
JURY IN RESPECT OF ANY LITIGATION BASED HEREON, OR DIRECTLY OR INDIRECTLY AT ANY
TIME ARISING OUT OF, UNDER OR IN CONNECTION WITH THE BRIDGE LOAN DOCUMENTS OR
ANY TRANSACTION CONTEMPLATED THEREBY OR ASSOCIATED THEREWITH, BEFORE OR AFTER
MATURITY; (B) WAIVES, TO THE MAXIMUM EXTENT NOT PROHIBITED BY LAW, ANY RIGHT IT
MAY HAVE TO CLAIM OR RECOVER IN ANY SUCH LITIGATION ANY "SPECIAL DAMAGES", AS
DEFINED BELOW, (C) CERTIFIES THAT NO PARTY HERETO NOR ANY REPRESENTATIVE OF NOR
COUNSEL FOR ANY PARTY HERETO HAS REPRESENTED, EXPRESSLY OR OTHERWISE, OR IMPLIED
THAT ANY PARTY HERETO WOULD NOT, IN THE EVENT OF LITIGATION, SEEK TO ENFORCE THE
FOREGOING WAIVERS, AND (D) ACKNOWLEDGES THAT IT HAS BEEN INDUCED TO ENTER INTO
THIS AGREEMENT, THE OTHER BRIDGE LOAN DOCUMENTS AND THE TRANSACTIONS
CONTEMPLATED HEREBY AND THEREBY BY, AMONG OTHER. THINGS, THE MUTUAL WAIVERS AND
CERTIFICATIONS CONTAINED IN THIS SECTION. AS USED IN THIS SECTION, "SPECIAL
DAMAGES" INCLUDES ALL SPECIAL, CONSEQUENTIAL, EXEMPLARY, OR PUNITIVE DAMAGES
(REGARDLESS OF HOW NAMED), BUT DOES NOT INCLUDE ANY PAYMENTS OR FUNDS WHICH ANY
PARTY HERETO HAS EXPRESSLY PROMISED TO PAY OR DELIVER TO ANY OTHER PARTY HERETO.
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 17


       If the foregoing correctly sets forth our mutual agreement, please so
acknowledge by signing and returning this Agreement to the undersigned.

                                        Very truly yours,

                                        NATIONSBANK, N. A.

                                        By:
                                           __________________________________
                                           Name: 
                                           Title:

                                        Bank's Address:

                                        901 Main Street
                                        Dallas, Texas 75202

                                        COPIES OF ALL NOTICES MUST BE SENT TO:

                                        NationsBank N. A.
                                        370 17th Street
                                        Denver, Colorado 80202
                                        Attention: David C. Rubenking
                                        Tel: 303/629-6969
                                        Fax: 303/629-6303
<PAGE>
 
Western Gas Resources, Inc.
February 17, 1999
Page 18


ACCEPTED as of the date first written above.

WESTERN GAS RESOURCES, INC.

By: /s/ William Krysiak
   -----------------------------
Name:  WILLIAM KRYSIAK
Title: V.P. FINANCE

1200 N. Pecos Street, Suite 230
Denver, Colorado 80234
Tel: 303/450-8439
Fax: 303/252-3362
<PAGE>
 
                                                                         ANNEX I
                                                                         -------

                                  DEFINITIONS
                                  -----------

     "Adjusted Base Rate" means the rate per annum equal to the Base Rate plus
      ------------------
the Base Rate Spread. The Adjusted Base Rate shall in no event, however, exceed
the Highest Lawful Rate.

     "Adjusted Eurodollar Rate" means, with respect to each particular
      ------------------------
Eurodollar Portion of the Bridge Loan and the associated Eurodollar Rate and
Reserve Percentage, the rate per annum calculated by Agent (rounded upwards, if
necessary, to the next higher 0.01%) determined on a daily basis pursuant to the
following formula:

Adjusted Eurodollar Rate =

Eurodollar Rate                 +  Eurodollar Spread
----------------------------
100.0% - Reserve Requirement

Such Adjusted Eurodollar Rate shall change as the Eurodollar Spread and the
associated Reserve Requirement change. The Adjusted Eurodollar Rate shall in no
event, however, exceed the Highest Lawful Rate.

     "Base Rate" means the per annum rate of interest equal to the greater of
      ---------
(i) the Prime Rate from time to time in effect or (ii) the Federal Funds Rate
from time to time in effect, plus one-half of one percent (.50%). If the Prime
Rate or the Federal Funds Rate, as the case may be, changes after the date
hereof the Base Rate shall be automatically increased or decreased, as the case
may be, without notice to Borrower from time to time as of the effective time of
each such change.

     "Base Rate Portion" means that portion of the unpaid principal balance of
      -----------------
the Bridge Loan which is not made up of Eurodollar Portions.

     "Base Rate Spread" means:
      ----------------

     (a)  from the date of the Agreement until and including April 31, 1999,
          0.50%;

     (b)  from and including May 1, 1999 until and including, July 31, 1999,
          1.00%; and

     (c)  from and including August 1, 1999 until and including the Maturity
          Date, 1.50%.

     "Bridge Loan Documents" means the Agreement, the Bridge Note, all
      ---------------------
guaranties of Borrower's Subsidiaries required to be delivered under the
Agreement, and the other agreements,
<PAGE>
 
instruments, and documents evidencing, securing, governing, guaranteeing, and/or
pertaining to the Bridge Loan.

     "Bridge Loan Obligations" means all Debt from time to time owing by
      -----------------------
Borrower or any other Person under or pursuant to any of the Bridge Loan
Documents. "Bridge Loan Obligation" means any part of the Bridge Loan
Obligations.

     "Default" means any Event of Default as described in Section 16 and any
      -------
default, event or condition which would, with the giving of any requisite
notices and the passage of any requisite periods of time, constitute an Event of
Default.

     "Default Rate" means, at the time in question, four percent (4.0%) per
      ------------
annum. plus the Adjusted Base Rate. The Default Rate shall in no event, however,
exceed the Highest Lawful Rate.

     "Eurodollar Portion" means any portion of the unpaid principal balance
      ------------------
of the Bridge Loan which Borrower designates as such in a Rate Election.

     "Eurodollar Rate" means, for any Eurodollar Portion and with respect to
      ---------------
the related Interest Period therefor, the rate per annum. (rounded upwards, if
necessary, to the nearest 1/100 of 1%) appearing on the Dow Jones Market Service
(formerly Telerate Access Service) Page 3750 (or any successor page) as the
London interbank offered rate for deposits in Dollars at approximately 11:00
a.m. (London time) two Business Days prior to the first day of such Interest
Period for a term comparable to such Interest Period. If for any reason such
rate is not available, the term "Eurodollar Rate" shall mean, for any Eurodollar
Portion and with respect to the related Interest Period therefor, the rate per
annum (rounded upwards, if necessary, to the nearest 1/100 of 1%) appearing on
Reuters Screen LIBO Page as the London interbank offered rate for deposits of
Dollars at approximately 11:00 a.m. (London time) two Business Days prior to the
first day of such Interest period for a term comparable to such Interest Period;
provided, however, if more than one rate is specified on Reuters Screen LIBO
Page, the applicable rate shall be the arithmetic mean of all such rates
(rounded upwards, if necessary, to the nearest 1/100 of 1%); to Bank or shall be
calculated by bank by a substantially similar methodology as that theretofore
used to determine such rate on Reuters Screen LIBO Page.

     "Eurodollar Rate Payment Date" means, with respect to any Eurodollar
      ----------------------------
Portion: (i) the day on which the related Interest Period ends and (ii) any day
on which past due interest or past due principal is owed hereunder with respect
to such Bridge Loan and is unpaid. If the terms of the Bridge Loan Documents
provide that payments of interest or principal with respect to such Bridge Loan
shall be deferred from one Eurodollar Rate Payment Date to another day, such
other day shall also be a Eurodollar Rate Payment Date.

                                       2
<PAGE>
 
     "Eurodollar Spread" means:
      -----------------

     (a)  from the date of the Agreement until and including May 31, 1999,
          1.75%;

     (b)  from and including June 1, 1999 until and including July 31, 1999,
          2.25%; and

     (c)  from and including August 1, 1999 until and including the Maturity
          Date, 2.75%.

     "Fair Market Value" of any property or any portion thereof or interest
      -----------------
therein, as of any date, means the after-tax cash proceeds that would be
obtained in an arms' length transaction for the sale thereof consummated on such
date between an informed and willing buyer and an informed and willing seller,
under no compulsion, respectively, to buy or sell, assuming that such property
has been maintained and operated in all material respects in accordance with the
requirements of the Revolving Credit Agreement, such price to be as agreed
between Borrower and Bank or, if Borrower and Bank are unable to agree, as
determined by a mutually acceptable qualified third-party appraiser.

     "Guarantor " means any other Person who has guaranteed some or all of
      ---------
the Bridge Loan

     "Highest Lawful Rate" means, with respect to Bank, the maximum
      -------------------
nonusurious rate of interest that Bank is permitted under applicable law to
contract for, take, charge, or receive with respect to the Bridge Loan.

     "Interest Period" means, with respect to each particular Eurodollar
      ---------------
Portion, a period of 1, 2 or 3 months, as specified in the Rate Election
applicable thereto, beginning on and including the date specified in such Rate
Election (which must be a Business Day), and ending on but not including the
same day of the month as the day on which it began (e.g., a period beginning on
the third day of one month shall end on but not. include the third day of
another-month), provided that each Interest Period which would otherwise end on
a day which is not a Business Day shall end on the next succeeding Business Day
(unless such next succeeding Business Day is the first Business Day of a
calendar month, in which case such Interest Period shall end on the immediately
preceding Business Day). No Interest Period may be elected, which would extend
past the date on which the Bridge Note is due and payable in fall.

     "Maturity Date" means October 31, 1999.
      -------------

     "Person" means an individual, corporation, partnership, limited liability
      ------
company, association, joint stock company, trust or trustee thereof, estate or
executor thereof, unincorporated organization or joint venture, court or
governmental unit or any agency or subdivision thereof, or-any other legally
recognizable entity.

                                       3
<PAGE>
 
     "Prime Rate" means the per annum rate of interest established from time
      ----------
to time by Bank as its prime rate, which rate may not be the lowest rate of
interest charged by Bank to its customers.

     "Rate Election" has the meaning given it in Section 2.
      -------------

     "Related Person" means any of Borrower, each Guarantor, and each other
      --------------
Subsidiary of Borrower, with the exception of Williston Gas Company, Westana,
and Sandia.

     "Reserve Requirement" means, on any day with respect to the Bridge Loan,
      -------------------
the maximum rate at which reserves (including, without limitation, any marginal,
special, supplemental, or emergency reserves) are required to be maintained
under regulations issued from time to time by the Board of Governors of the
Federal Reserve System (or any successor) by member banks of the Federal Reserve
System against "Eurocurrency liabilities" (as such term is used in Regulation
D). Without limiting the effect of the foregoing, the Reserve Requirement shall
reflect any other reserves required to be maintained by such member banks with
respect to (i) any category of liabilities which includes deposits by reference
to which the Adjusted Eurodollar Rate is to be determined, or (ii) any category
of extensions of credit or other assets which include the Bridge Loan. The
Adjusted Eurodollar Rate shall be adjusted automatically on and as of the
effective date of any change in the Reserve Requirement.

     "Revolving Credit Agreement" means that certain Credit Agreement dated as
      --------------------------
of May 30, 1997 among Borrower and NationsBank, N.A., successor in interest by
merger to NationsBank of Texas, N.A., as Agent, and certain other financial
institutions, as Lenders, as from time to time amended, supplemented, or
restated.

     "Subsidiary" means, with respect to any Person, any corporation,
      ----------
association, partnership, joint venture, or other business or corporate entity,
enterprise or organization which is directly or effectively through one or more
intermediaries, controlled by or owned fifty-one percent or more by such Person,
provided that associations, joint ventures or other relationships (a) which are
established pursuant to a standard form operating agreement or similar agreement
or which are partnerships for purposes of federal income taxation only, (b)
which are not corporations or partnerships (or subject to the Uniform
Partnership Act) under applicable state law, and (c) whose businesses are
limited to the exploration, development and operation of oil, gas, mineral, gas
gathering or gas processing properties and interests owned directly by the
parties in such associations, joint ventures or relationships, shall not be
deemed to be "Subsidiaries" of such Person.

                                       4
<PAGE>
 
                                                                       Exhibit A
                                                                       ---------

                                PROMISSORY NOTE

$37,000,000                      Dallas, Texas              February _, 1999

     FOR VALUE RECEIVED, the undersigned, Western Gas Resources Inc., a Delaware
corporation (herein called "Borrower"), hereby promises to pay to the order of
NATIONSBANK, N.A. ("Bank"), the principal sum of Thirty-Seven Million Dollars
($37,000,000), or, if greater or less, the aggregate unpaid principal amount of
the Bridge Loan made under this Note by Bank to Borrower pursuant to the terms
of the Agreement (as hereinafter defined), together with interest on the unpaid
principal balance thereof as hereinafter set forth, both principal and interest
payable as herein provided in lawful money of the United States of America at
the offices of Bank, 901 Main Street, Dallas, Texas or at such other place
within Dallas County, Texas, as from time to time may be designated by the
holder of this Note.

     This Note (i) is issued and delivered under that certain Agreement dated as
of February 17, 1999, among Borrower and Bank (herein, as from time to time
supplemented, amended or restated, called the "Agreement"), and is the "Bridge
Note" as defined therein, (ii) is subject to the terms and provisions of the
Agreement, which contains provisions for payments and prepayments hereunder and
acceleration of the maturity hereof upon the happening of certain stated events,
and (iii) is secured by and entitled to the benefits of certain guaranties.
Payments on this Note shall be made and applied as provided herein and in the
Agreement. Reference is hereby made to the Agreement for a description of
certain rights, limitations of rights, obligations and duties of the parties
hereto and for the meanings assigned to terms used and not defined herein.

     The principal amount of this Note. shall be due and payable in full on the
Maturity Date. This Note is subject to mandatory prepayments as required in the
Agreement.

     The Base Rate Portion of the Bridge Loan (exclusive of any past due
principal or interest) from time to time outstanding shall bear interest on each
day outstanding at the Adjusted Base Rate in effect on such day; provided that
if an Event of Default has occurred and is continuing, the Base Rate Portion of
the Bridge Loan (exclusive of any past due principal or interest) from time to
time outstanding shall bear interest on each day outstanding at the Default Rate
in effect on such day. On each Base Rate Payment Date, Borrower shall pay to the
holder hereof all unpaid interest which has accrued on the Base Rate Portion to
but not including such Base Rate Payment Date.

     Each Eurodollar Portion of the Bridge Loan (exclusive of any past due
principal or interest) shall bear interest on each day during the related
Interest Period at the related Adjusted Eurodollar Rate in effect on such day;
provided that if an Event of Default has occurred and is continuing, each
Eurodollar Portion of the Bridge Loan (exclusive of any past due principal or
<PAGE>
 
interest) shall bear interest on each day during the related Interest Period at
the Default Rate in effect on such day. On each Eurodollar Rate Payment Date,
Borrower shall pay to the holder hereof all unpaid interest which has accrued on
such Eurodollar Portion to but not including such Eurodollar Rate Payment Date.

     All past due principal of and past due interest on the Bridge Loan shall
bear interest on each day outstanding at the Default Rate in effect on such day,
and such interest shall be due and payable immediately as it accrues.

     Notwithstanding the foregoing paragraph and all other provisions of this
Note, in no event shall the interest rate payable hereon, whether before or
after maturity, exceed the Highest Lawful Rate, and this Note is expressly made
subject to the provisions of the Agreement which more fully set out the
limitations on how interest accrues hereon. In the event applicable law provides
for a ceiling under Chapter 303 of the Texas Finance Code, as amended (the
"Texas Finance Code"), for that day, that ceiling shall be the "weekly" ceiling
as defined in the Texas Finance Code and shall be used in this Note for
calculating the Highest Lawful Rate and for all other purposes. The term
"applicable law" as used in this Note shall mean the laws of the State of Texas
or the laws of the United States, whichever laws allow the greater interest, as
such laws now exist or may be changed or amended or come into effect in the
future.

     If this Note is placed in the hands of an attorney for collection after
default, or if all or any part of the indebtedness represented hereby is proved,
established or collected in any court or in any bankruptcy, receivership, debtor
relief, probate or other court proceedings, Borrower and all endorsers, sureties
and guarantors of this Note jointly and severally agree to pay reasonable
attorneys' fees and collection costs to the holder hereof in addition to the
principal and interest payable hereunder.

     Borrower and all endorsers, sureties and guarantors of this Note hereby
severally waive demand, presentment for payment, protest, notice of protest,
notice of intention to accelerate the maturity of this Note, notice of the
acceleration of the maturity of this Note, diligence in collecting, the bringing
of any suit against any party and any notice of or defense on account of any
extensions, renewals, partial payments or changes in any manner of or in this
Note or in any of its terms, provisions and covenants, or any releases or
substitutions of any security, or any delay, indulgence or other act of any
trustee or any holder hereof, whether before or after maturity.

                                       2
<PAGE>
 
     THIS NOTE AND THE RIGHTS, DUTIES AND LIABILITIES OF THE PARTIES HEREUNDER
AND/OR ARISING FROM OR RELATING IN ANY WAY TO THE INDEBTEDNESS EVIDENCED BY THIS
NOTE OR THE TRANSACTION OF WHICH SUCH INDEBTEDNESS IS A PART SHALL BE GOVERNED
AND CONSTRUED FOR ALL PURPOSES BY THE LAWS OF THE STATE OF TEXAS, EXCEPT TO THE
EXTENT THE SAME ARE GOVERNED BY APPLICABLE FEDERAL LAW.

                                          WESTERN GAS RESOURCES, INC.

                                          By:__________________________________
                                             Name:
                                             Title:

                                       3
<PAGE>
 
                                                                       Exhibit B
                                                                       ---------

                                 RATE ELECTION

     Reference is made to that certain Agreement dated as of February 17, 1999
(as from Time to time amended, the "Agreement"), by and among Western Gas
                                    ---------
Resources, Inc. ("Borrower") and NationsBank, N.A. Terms which are defined in
                  --------
the Agreement and which are used but not defined herein are used herein with the
meanings given them in the Agreement. Pursuant to the terms of the Agreement
Borrower hereby elects a Eurodollar Portion in the aggregate amount of
$_________ with an Interest Period beginning on _______________ and continuing
for A period of ______________.

     The undersigned officer of Borrower hereby certifies that he is a duly
elected, qualified and acting officer of Borrower, having all necessary
authority to act in making the election herein contained.

     IN WITNESS WHEREOF this instrument is executed as of ___________, 1999.


                                        WESTERN GAS RESOURCES, INC.

                                        By:________________________________
                                           Name:
                                           Title: