Document:

<PAGE>

                                                                   EXHIBIT 10.37

                                                                  CONFORMED COPY
                                                                  --------------

                           DATE: 23RD DECEMBER, 1999

                            CROWN CASTLE UK LIMITED
                                  AS BORROWER

                       CROWN CASTLE UK HOLDINGS LIMITED
                                      AND
                       MILLENNIUM COMMUNICATIONS LIMITED
                                 AS GUARANTORS

                       THE LENDERS LISTED IN SCHEDULE 1

                          CREDIT SUISSE FIRST BOSTON
                                   AS AGENT

               ------------------------------------------------
                           LOAN AMENDMENT AGREEMENT
               ------------------------------------------------

                               SLAUGHTER AND MAY
                             35 BASINGHALL STREET
                                    LONDON
                                   EC2V 5DB

                                   (AGB/HZM)
<PAGE>

                           LOAN AMENDMENT AGREEMENT

DATE: 23rd December, 1999

PARTIES

1.   CROWN CASTLE UK LIMITED (formerly known as Castle Transmission
     International Ltd.), a company incorporated in England (number 3196207), of
     Warwick Technology Park, Gallows Hill, Heathcote Lane, Warwick CV34 6TN, as
     borrower

2.   CROWN CASTLE UK HOLDINGS LIMITED (formerly known as Castle Transmission
     Services (Holdings) Ltd.), a company incorporated in England (number
     3242381), of Warwick Technology Park, Gallows Hill, Heathcote Lane, Warwick
     CV34 6TN and MILLENNIUM COMMUNICATIONS LIMITED, a company incorporated in
     England (number 2903056), of Warwick Technology Park, Gallows Hill,
     Heathcote Lane, Warwick CV34 6TN, as guarantors

3.   CREDIT SUISSE FIRST BOSTON, for itself and as agent for the Lenders (the
     "Agent")

BACKGROUND

(A)  On 28th February, 1997 the loan agreement was entered into between the
     Borrower, the Parent, certain lenders, the Agent and others and under its
     terms the lenders agreed to provide term and revolving loan facilities of
     (Pounds)162,500,000 to the Borrower.  On 21st May, 1997 the parties to the
     loan agreement at that date amended the loan agreement so that the lenders
     under it agreed to provide revolving loan facilities of (Pounds)64,000,000
     to the Borrower.  These loan facilities are guaranteed by the Parent and
     secured by charges granted by the Borrower and the Parent.  With effect
     from 27th October, 1998 Millennium acceded to the loan agreement as an
     additional guarantor and acceded to the Debenture as an additional chargor.
     The loan agreement was further amended on 18th June, 1999, amongst other
     things, from a (Pounds)64,000,000 revolving loan facility to a
     (Pounds)150,000,000 term and revolving loan facility.

(B)  At the request of the Borrower the parties have agreed to amend the terms
     of the Loan Agreement on the terms of this Agreement.

(C)  The Agent has been authorised by an Instructing Group to agree to the
     amendments contained in this Agreement.

The parties agree as follows:

1.   INTERPRETATION

1.1  Loan Agreement

     The interpretation provisions contained in Part I of the Loan Agreement are
     deemed to be incorporated expressly in this Agreement, and apply to this
     Agreement accordingly.
<PAGE>

1.2  Definition

     In this Agreement "Loan Agreement" means the loan agreement dated 28th
     February, 1997 as amended on 21st May, 1997, acceded to by Millennium with
     effect from 27th October, 1998 and as further amended on 18th June, 1999
     made between the Borrower, the Parent, Millennium, the Lenders, Credit
     Suisse First Boston and others under which the Lenders have agreed to
     provide (Pounds)150,000,000 term and revolving loan facilities to the
     Borrower.

1.3  Scope

     This Agreement is supplemental to and amends the Loan Agreement.

2.   AMENDMENT OF THE LOAN AGREEMENT

     The parties agree that the Loan Agreement shall be amended with effect from
     the date of this Agreement so that the reference in each of Clauses
     20.1(EE), 21.1(L)(iii)(b) and 21.1(M)(iii)(b) to "16th July, 1999" shall be
     replaced by a reference in each of these Clauses to "31st December, 1999".

3.   MISCELLANEOUS

3.1  Law

     This Agreement is to be governed by and construed in accordance with
     English law.

3.2  Counterparts

     There may be several signed copies of this Agreement.  There is intended to
     be a single Agreement and each signed copy is a counterpart of that
     Agreement.
<PAGE>

                                  SIGNATURES

Borrower

CROWN CASTLE UK LIMITED

By:

Parent and Guarantor

CROWN CASTLE UK HOLDINGS LIMITED

By:

Additional Guarantor

MILLENNIUM COMMUNICATIONS LIMITED

By:

Agent

CREDIT SUISSE FIRST BOSTON

By:<PAGE>

                                                                   EXHIBIT 10.23

                     AMENDED AND RESTATED CREDIT AGREEMENT

     THIS AMENDED AND RESTATED CREDIT AGREEMENT (this "Amendment") dated as of
the 1/st/ day of December, 1999, by and among ALL AMERICAN PIPELINE, L.P.
("Borrower"), PLAINS MARKETING, L.P. ("Marketing"), PLAINS ALL AMERICAN
PIPELINE, L.P. ("Plains MLP"), and BANKBOSTON, N.A., as Administrative Agent (in
such capacity, "Administrative Agent"), and the Lenders party hereto.

                             W I T N E S S E T H:

     WHEREAS, Borrower, Marketing, Plains MLP, Administrative Agent, and Lenders
entered into that certain Credit Agreement dated as of November 17, 1998 (as
amended, restated, or supplemented to the date hereof, the "Original Agreement")
for the purposes and consideration therein expressed, pursuant to which Lenders
became obligated to make and made loans to Borrower as therein provided; and

     WHEREAS, Borrower, Marketing, Plains MLP, Administrative Agent, and Lenders
desire to amend and restate 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, as amended and
restated hereby, 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; Restatement
                           ---------------------------------------

     (S) 1.1.  Terms Defined in the Original Agreement.  Unless the context
               ---------------------------------------
otherwise requires or unless otherwise expressly defined herein, the terms
defined in the Original Agreement, as amended and restated hereby, shall have
the same meanings whenever used herein.

     (S) 1.2.  Other Defined Terms.  Unless the context otherwise requires, the
               -------------------
following terms when used herein shall have the meanings assigned to them in
this (S) 1.2.

               "Amendment" means this Amended and Restated Credit Agreement.
                ---------

               "Credit Agreement" means the Original Agreement as amended and
                ----------------
     restated hereby.

     (S) 1.3.  Restatement.  The Original Agreement is hereby restated in its
               -----------
current form with the amendments set forth herein below. This Amendment,
together with the Original Agreement, shall be deemed to be an amendment and
restatement of the Original Agreement. All references to the Credit Agreement in
any other document, instrument, agreement, or writing shall hereafter
<PAGE>

be deemed to refer to this Amendment and to the Original Agreement as amended
and restated hereby.

     (S) 2.1.  Definitions.
               -----------

     (a)       Modifications.  References to the "Marketing Credit Agreement"
               -------------
contained in the Original Credit Agreement shall be deemed to refer to such
agreement as amended from time to time, including as of the date hereof. The
definitions of Base Rate, Consolidated EBITDA, Consolidated Funded Indebtedness,
Default Rate, Interest Expense, Letter of Credit Fee Rate, Revolver Eurodollar
Rate Margin, and Term Loan Eurodollar Rate Margin contained in Section 1.1 of
the Original Agreement are hereby amended in their entirety to read as follows:

               "Base Rate' means the sum of (a) the Base Rate Margin plus (b)
                ---------
     the higher of (i) the annual rate of interest announced from time to time
     by Administrative Agent at its "base rate" at its head office in Boston,
     Massachusetts, or (ii) the Federal Funds Rate plus one-half percent (0.5%)
     per annum; provided that such rate may not be the lowest rate at which
     funds are made available to customers of Administrative Agent at such time.
     Each change in the Base Rate shall become effective without prior notice to
     Borrower automatically as of the opening of business on the date of such
     change in the Base Rate."

               "Consolidated EBITDA' means, for any four-Fiscal Quarter period,
                -------------------
     the sum of (1) the Consolidated Net Income of Plains MLP and its
     Subsidiaries during such period, plus (2) all interest expense which was
     deducted in determining such Consolidated Net Income for such period, plus
     (3) all income taxes (including any franchise taxes to the extent based
     upon net income) which were deducted in determining such Consolidated Net
     Income, plus (4) all depreciation, amortization (including amortization of
     good will and debt issue costs) and other non-cash charges (including any
     provision for the reduction in the carrying value of assets recorded in
     accordance with GAAP) which were deducted in determining such Consolidated
     Net Income, plus (5) the Identified Loss Adjustment, minus (6) all non-cash
     items of income which were included in determining such Consolidated Net
     Income.  The term "Identified Loss Adjustment" means (i) for each four-
                        --------------------------
     Fiscal Quarter period ending on or prior to March 31, 2000, the portion of
     the Identified Loss incurred in such period not to exceed an aggregate
     amount of $180,000,000 and (ii) for each four-Fiscal Quarter period ending
     after March 31, 2000, zero."

               "Consolidated Funded Indebtedness' means as of any date, the
                --------------------------------
     sum of the following (without duplication): (i) all Indebtedness which is
     classified as "long-term indebtedness" on a consolidated balance sheet of
     Plains MLP and its Consolidated Subsidiaries prepared as of such date in
     accordance with GAAP and any current maturities or other principal amount
     in respect of such Indebtedness due within one year but which was
     classified as "long-term indebtedness" at the creation thereof, (ii)
     indebtedness for borrowed money of Plains MLP and its Consolidated
     Subsidiaries outstanding under a revolving credit or similar agreement
     providing for borrowings (and renewals and extensions thereof) over a
     period of more than one year, notwithstanding

                                      -2-
<PAGE>

     the fact that any such borrowing is made within one year of the expiration
     of such agreement, and (iii) Indebtedness in respect of Capital Leases of
     Plains MLP and its Consolidated Subsidiaries; provided, however,
     Consolidated Funded Indebtedness shall not include (i) Indebtedness in
     respect of letters of credit or in respect of Cash and Carry Purchases and
     (ii) subordinate Indebtedness referred to in subsection 7.1(g)."

               "Default Rate' means, at the time in question, (i) four percent
                ------------
     (4%) per annum plus the Adjusted Eurodollar Rate then in effect for any
     Eurodollar Loan (up to the end of the applicable Interest Period) or (ii)
     two percent (2%) per annum plus the Base Rate for each Base Rate Loan;
     provided, however, the Default Rate shall never exceed the Highest Lawful
     Rate."

               "Interest Expense' means, with respect to any period, the sum
                ----------------
     (without duplication) of the following (in each case, eliminating all
     offsetting debits and credits between Plains MLP and its Subsidiaries and
     all other items required to be eliminated in the course of the preparation
     of Consolidated financial statements of Plains MLP and its Subsidiaries in
     accordance with GAAP): (a) all interest and commitment fees in respect of
     Indebtedness of Plains MLP or any of its Subsidiaries (including imputed
     interest on Capital Lease Obligations) which are accrued during such period
     and whether expensed in such period or capitalized; plus (b) all fees,
     expenses and charges in respect of letters of credit issued for the account
     of Plains MLP or any of its Subsidiaries, which are accrued during such
     period and whether expensed in such period or capitalized. Interest which
     is accrued but unpaid on the subordinate Indebtedness referred to in
     subsection 7.1(g), the fees payable pursuant to the letter agreement
     described in Section 2.12(d)(ii) of the Marketing Agreement, and the
     amendment fee payable pursuant to Section 3.1(f) of the Third Amendment to
     the Credit Agreement shall not be treated as Interest Expense.

               "Letter of Credit Fee Rate' means two and one-quarter percent
                -------------------------
     (2.25%) per annum."

               "Revolver Eurodollar Rate Margin' means two and one-quarter
                -------------------------------
     percent (2.25%) per annum."

               "Term Loan Eurodollar Rate Margin' means two and one-quarter
                --------------------------------
     percent (2.25%) per annum."

     (b)       Additions.  The following definitions are hereby added to
               ---------
Section 1.1 of the Original Agreement:

               "Base Rate Margin' means three-quarters of one percent (0.75%)
                ----------------
     per annum."

               "Identified Loss' means (i) the loss in the aggregate amount of
                ---------------
     $160,000,000 arising from unauthorized trading activity by Marketing during
     the period of January 1, 1999 to November 20, 1999 plus (ii) the fees and
     expenses of lawyers, accountants and

                                      -3-
<PAGE>

     other professionals and fees associated with amendments and waivers by
     lenders, in each case associated with such loss in an aggregate amount up
     to $20,000,000."

     (c)       Deletions.  The definitions of "Applicable Leverage Ratio",
               ---------
"Applicable Rating Level", and "Term Loan Base Rate Margin" are hereby deleted
in their entirety from the Original Agreement.

     (S) 2.2.  Interest Rates and Fees.  The reference to "Base Rate plus the
               -----------------------
Term Loan Base Rate Margin" in Section 2.5(b) of the Original Agreement is
hereby deemed to read "Base Rate".

     (S) 2.3.  Indebtedness.
               ------------

     (a)       Section 7.1(e) of the Original Agreement is hereby amended in its
entirety to read as follows:

               "(e)  Indebtedness under the Marketing Credit Agreement, provided
     that the principal amount of loans and face amount of letters of credit
     thereunder at any one time outstanding shall not exceed $375,000,000;"

     (b)       Section 7.1 of the Original Agreement is hereby amended by
renumbering clause (g) to be clause (h) and to insert a new clause (g), to read
as follows:

               "(g)  Indebtedness in a principal amount not at any time in
     excess of $114,000,000 (plus accrued unpaid interest on such principal
     amount) owing by Marketing to General Partner and subordinated to the
     Obligations under this Agreement and the Indebtedness under the Marketing
     Agreement on terms and conditions satisfactory to Administrative Agent."

     (S) 2.4.  Limitation on Dividends and Redemptions. Section 7.6 of the
               ---------------------------------------
Original Agreement is hereby to read as follows:

               "Section 7.6  Limitation on Dividends and Redemptions.  No
                             ---------------------------------------
     Restricted Person will declare or pay any dividends on, or make any other
     distribution in respect of, any class of its capital stock or any
     partnership or other interest in it, nor will any Restricted Person
     directly or indirectly make any capital contribution to or purchase,
     redeem, acquire or retire any shares of the capital stock of or partnership
     interests in any Restricted Person (whether such interests are now or
     hereafter issued, outstanding or created), or cause or permit any reduction
     or retirement of the capital stock of any Restricted Person, while any Loan
     is outstanding. Notwithstanding the foregoing, but subject to Section 7.5,
     (i) Subsidiaries of Plains MLP, Borrower, or of any Guarantor shall not be
     restricted, directly or indirectly, from declaring and paying dividends or
     making any other distributions to Plains MLP, Borrower, or any such
     Guarantor, respectively, (ii) no Restricted Person shall be restricted from
     making capital contributions to a Wholly Owned Subsidiary of such
     Restricted Person that is a Guarantor, and (iii) Plains MLP may make a
     regular quarterly distribution of Available

                                      -4-
<PAGE>

     Cash to its partners in accordance with the Partnership Agreement if, and
     only if, Majority Lenders shall have given their prior written consent to
     such distribution."

     (S) 2.5.  Current Ratio. Section 7.11 of the Original Agreement is hereby
amended in its entirety to read as follows:

               "Section 7.11.  Current Ratio. The ratio of (i) the sum of Plains
     MLP's Consolidated current assets plus the excess, if any, of the Revolver
     Commitment over the Revolver Usage to (ii) Plains MLP's Consolidated
     current liabilities will never be less than 1.0 to 1.0. For purposes of
     this section, Plains MLP's Consolidated current liabilities will be
     calculated without including (a) any payments of principal on the Notes
     which are required to be repaid within one year from the time of
     calculation and (b) all Liabilities arising under permitted Hedging
     Contracts. Further, Consolidated current liabilities shall be decreased by
     (x) $105,000,000 for the period from December 1, 1999 to and including
     January 31, 2000 and (y) $10,000,000 for the period from January 31, 2000
     through and including February 28, 2000."

     (S) 2.6.  Debt to Capital Ratio.  Section 7.15 of the Original Agreement is
               ---------------------
hereby amended in its entirety to read as follows:

               "Section 7.15  Debt to Capital Ratio.  On or before April 30,
                              ---------------------
     2000, the ratio of (a) all Consolidated Funded Indebtedness to (b) the sum
     of Consolidated Funded Indebtedness plus Consolidated Net Worth plus one-
     half (50%) of the Identified Loss will never be greater than .60 to 1.0 at
     any time. After April 30, 2000, the ratio of (a) all Consolidated Funded
     Indebtedness to (b) the sum of Consolidated Funded Indebtedness plus
     Consolidated Net Worth will never be greater than .60 to 1.0 at any time."

     (S) 2.7.  Open Inventory Position.  Section 7.16 is hereby added to the
Original Agreement to read as follows:

               "Section 7.16.  Open Inventory Position. Borrower shall not at
     any time have an Open Position other than inventory consisting of tank
     bottoms, pipeline line fill requirements and other working inventory not to
     exceed at any time 600,000 barrels in the aggregate."

     (S) 2.8.  Waiver.  This Amendment, upon its effectiveness as provided in
               ------
(S) 3.1, shall waive (i) the violations of Sections 7.6, 7.11, and 7.15 of the
Original Agreement by Borrower on and prior to the date hereof, based upon
unauthorized trading activities disclosed by Plains MLP in its press release of
November 29, 1999, (ii) the Events of Default arising under Sections 8.1(f) and
8.1(g) of the Original Agreement on and prior to the date hereof, based upon
unauthorized trading activities disclosed by Plains MLP in its press release of
November 29, 1999, and (iii) those Defaults and Events of Default occurring on
or before December 1, 1999 resulting from the foregoing.

                                      -5-
<PAGE>

     (S) 2.9.  Consent.  Each Lender a party hereto hereby consents to (i) the
               -------
sale by Borrower of the linefill carried in the All American Pipeline during the
period from December 1, 1999 through February 29, 2000 as is contracted for on
the date of this Amendment, (ii) any earlier monetization of such linefill and
the contracts for the sale thereof (including, without limitation, a sale or
loan with recourse limited to such linefill and contracts), (iii) the retention
of the proceeds of such sale or monetization in a cash collateral account within
the control of Administrative Agent under the Credit Agreement to secure, on a
first priority basis, the Obligations and, on a second priority basis, the
Indebtedness under the Marketing Credit Agreement, in accordance with the
Intercreditor Agreement, (iv) the delivery of such proceeds to Marketing by the
Administrative Agent from time to time upon request by Marketing for the payment
of accounts payable of Marketing on the date such payables are actually paid and
(v) the release of the Lien under the Security Documents on such linefill and
such proceeds upon use of such proceeds for such purpose. The Restricted Persons
agree that the portion of the transferred proceeds owned by Borrower shall be
made as a loan from Borrower to Marketing, and that such loan shall evidenced in
a manner satisfactory to the Administrative Agent and shall be subordinated to
the Obligations under the Credit Agreement and to the Indebtedness under the
Marketing Credit Agreement on terms and conditions satisfactory to the
Administrative Agent.

                  ARTICLE III. -- Conditions of Effectiveness
                                  ---------------------------

     (S) 3.1.  Effective Date.  This Amendment shall become effective as of the
               --------------
date first above written when and only when:

               (a) Administrative Agent shall have received, at Administrative
     Agent's office: (i) a counterpart of this Amendment executed and delivered
     by Borrower, Marketing, Plains MLP, Administrative Agent, and each Lender,
     (ii) a certificate of a duly authorized officer of General Partner (A)
     stating 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,
     (B) stating that no Material Adverse Change shall have occurred other than
     the Identified Loss, and (C) having attached thereto as an exhibit a copy
     of resolutions duly adopted by the Board of Directors of General Partner in
     full force and effect at the time that this Amendment is entered into, and
     containing such other certifications as shall be required by Administrative
     Agent, in form and substance acceptable to Administrative Agent, (iii) an
     opinion of Fulbright & Jaworski, L.L.P., special Texas and New York counsel
     to Restricted Persons, in form and substance acceptable to Administrative
     Agent in Administrative Agent's sole and absolute discretion, and (iv) an
     opinion of Michael R. Patterson, General Counsel for Restricted Persons.

               (b) General Partner shall have made on or after November 21, 1999
     a loan to Marketing in an amount not less than $64,000,000 which loan shall
     have been subordinated to the Obligations upon terms and conditions
     satisfactory to Administrative Agent.

               (c) Marketing shall have used its best efforts to have pledged
     all of the outstanding limited partnership interests in Plains Scurlock
     Permian, L.P. as Collateral

                                      -6-
<PAGE>

     pursuant to Security Documents satisfactory to Administrative Agent, such
     Collateral to be "Marketing Priority Collateral" (as such term is defined
     in the Intercreditor Agreement).

               (d) Payment of all fees and expenses of Thompson & Knight LLP,
     counsel to Administrative Agent.

               (e) A contemporaneous amendment to the Marketing Credit Agreement
     which has the effect of increasing the Maximum Facility Amount (as defined
     therein) to not less than $300,000,000 (as set forth on Schedule 3.1) but
     no more than $450,000,000 and making substantially the same amendments and
     waivers as set forth in (S)(S) 2.3 - 2.7 of this Amendment and amending the
     definition of Consolidated EBITDA as set forth in (S) 2.1 of this
     Amendment.

               (f) The payment to each Lender of an amendment fee of .375% of
     the sum of such Lender's Revolver Commitment plus such Lender's Term Loans.

               (g) Administrative Agent shall have received all documents and
     instruments which Administrative Agent has then requested, in addition to
     those described in the foregoing (S) 3.1(a) through and including (S)
     3.1(f).

                 ARTICLE IV. -- Representations and Warranties
                                ------------------------------

     (S) 4.1.  Representations and Warranties of Plains MLP and Borrower.  In
               ---------------------------------------------------------
order to induce Administrative Agent and Lenders to enter into this Amendment,
Plains MLP and Borrower represent and warrant to Administrative Agent and each
Lender that:

               (a)  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 to the extent that such representation and
     warranty was made as of a specific date.

               (b) Each Restricted Person is duly authorized to execute and
     deliver this Amendment, and Borrower is and will continue to be duly
     authorized to borrow and perform its obligations under the Credit
     Agreement. Each Restricted Person has duly taken all corporate action
     necessary to authorize the execution and delivery of this Amendment and to
     authorize the performance of their respective obligations hereunder.

               (c) The execution and delivery by each Restricted Person of this
     Amendment, the performance by each Restricted Person of its respective
     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 constituent documents of any
     Restricted Person, or of any material agreement, judgment, license, order
     or permit applicable to or binding upon any Restricted Person, or result in
     the creation of any lien, charge or encumbrance upon any assets or
     properties of any Restricted Person, except in favor of Administrative
     Agent for the benefit of Lenders and other Permitted Liens. Except for
     those which have been duly obtained, no consent, approval, authorization or

                                      -7-
<PAGE>

     order of any court or governmental authority or third party is required in
     connection with the execution and delivery by any Restricted Person of this
     Amendment or to consummate the transactions contemplated hereby.

               (d) When this Amendment has been duly executed and delivered,
     each of the Loan Documents, as amended by this Amendment, will be a legal
     and binding instrument and agreement of each Restricted Person, enforceable
     in accordance with its terms, (subject, as to enforcement of remedies, to
     applicable bankruptcy, insolvency and similar laws applicable to creditors'
     rights generally and to general principles of equity).

               (e) No Material Adverse Change has occurred other than the
     Identified Loss.

                          ARTICLE V. -- Miscellaneous
                                        -------------

                                      -8-
<PAGE>

     (S) 5.1.  Ratification of Agreements; Release.  The Original Agreement, as
               -----------------------------------
hereby amended, is hereby ratified and confirmed in all respects. The Loan
Documents (including but not limited to each Guaranty), as they may be amended
or affected by this Amendment, are hereby ratified and confirmed in all respects
by each Restricted Person to the extent a party thereto. Any reference to the
Credit Agreement in any Loan Document shall be deemed to refer to this Amendment
also. 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 Administrative Agent or any Lender under the Credit Agreement or any
other Loan Document nor constitute a waiver of any provision of the Credit
Agreement or any other Loan Document. As further consideration and to induce
each Lender Party to enter into and grant the accommodations contained in this
Amendment, each Restricted Person - on behalf of itself and, to the extent it is
permitted by law or is otherwise expressly authorized to do so, on behalf of all
of its affiliates (collectively, "Releasing Parties") -- hereby generally
release and forever discharge each Lender party and each of their respective
affiliates (collectively, "Released Parties"), from any and all claims, demands,
and causes of action of whatever kind or character which such Releasing party
has, or may have in the future, based on any actions, failures to act, or events
that have occurred prior to the effective date hereof, which in any way relate
to or are based upon (i) any transactions of any kind among the Releasing
Parties , on the one hand, and the Released Parties, on the other hand, or (ii)
any actual or alleged negotiations, discussions, representations, warranties,
promises, or other undertakings by Released Parties in connection with any of
the foregoing (the "Released Claims"). This release is to be construed as the
broadest type of general release and covers and releases any and all Released
Claims, whether known or unknown and however or whenever arising, whether by
contract or agreement, at law or under any statute (including without limitation
any law or statute pertaining to negligence, gross negligence, strict liability,
fraud, deceptive trade practices, negligent misrepresentation, securities
violations, breach of fiduciary duty, breach of contract, trade regulation,
regulation of business or competition, conspiracy or racketeering), or otherwise
arising, and expressly including any claims for punitive or exemplary damages,
attorneys' fees, or penalties. To the extent that any Released Claims with
respect to Released Parties have not been released by this letter agreement,
each Releasing Party hereby assign s such Released Claims to Released Parties.

     (S) 5.2.  Ratification of Security Documents.  Restricted Persons,
               ----------------------------------
Administrative Agent, and Lenders each acknowledge and agree that any and all
indebtedness, liabilities or obligations arising under or in connection with the
LC Obligations, as amended hereby, or the Notes, as

                                      -9-
<PAGE>

amended hereby, are Obligations and are secured indebtedness under, and are
secured by, each and every Security Document to which any Restricted Person is a
party. Each Restricted Person hereby re-pledges, re-grants and re-assigns a
security interest in and lien on every asset of the such Restricted Person
described as Collateral in any Security Document. Each Lender hereby
acknowledges and confirms that all Obligations under the Credit Agreement, as
amended hereby, shall be subject to the terms of the Intercreditor Agreement and
hereby acknowledges and agrees that the Collateral described in Section 3.1(c)
hereof shall be deemed "Marketing Priority Collateral" (as defined in the
Intercreditor Agreement).

     (S) 5.3.  Ratification of Intercreditor Agreement.  Each Lender hereby
               ---------------------------------------
acknowledges and confirms that all Obligations under the Credit Agreement, as
amended hereby, and the "Obligations" under the Marketing Credit Agreement, as
amended on the date hereof, shall be and shall remain subject to the terms and
entitled to the benefits of the Intercreditor Agreement.

     (S) 5.4.  Survival of Agreements.  All representations, warranties,
               ----------------------
covenants and agreements of the Restricted Persons herein shall survive the
execution and delivery of this Amendment and the performance hereof, including
without limitation the making or granting of each Loan, 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 any
Restricted Person hereunder or under the Credit Agreement to Administrative
Agent or any Lender shall be deemed to constitute representations and warranties
by, or agreements and covenants of, such Restricted Person under this Amendment
and under the Credit Agreement.

     (S) 5.5.  Loan Documents.  This Amendment is a Loan Document, and all
               --------------
provisions in the Credit Agreement pertaining to Loan Documents apply hereto.

     (S) 5.6.  GOVERNING LAW.  THIS AMENDMENT SHALL BE GOVERNED BY AND CONSTRUED
               -------------
IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK AND ANY APPLICABLE LAWS OF
THE UNITED STATES OF AMERICA IN ALL RESPECTS, INCLUDING CONSTRUCTION, VALIDITY
AND PERFORMANCE.

     (S) 5.7.  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.

                                     -10-
<PAGE>

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

                              ALL AMERICAN PIPELINE, L.P.

                              By:  PLAINS ALL AMERICAN INC.,
                                   its general partner

                                   By: /s/ Phil Kramer
                                       Phil Kramer
                                       Executive Vice President

                              PLAINS ALL AMERICAN PIPELINE, L.P.

                              By:   PLAINS ALL AMERICAN INC.,
                                    its general partner

                                    By: /s/ Phil Kramer
                                       -----------------------------------------
                                       Phil Kramer
                                       Executive Vice President

                              PLAINS MARKETING, L.P.

                              By:   PLAINS ALL AMERICAN INC.,
                                    its general partner

                                    By: /s/ Phil Kramer
                                       -----------------------------------------
                                       Phil Kramer
                                       Executive Vice President

                                    BANKBOSTON, N.A., as Administrative Agent
                                    and Lender

                                    By: /s/ T. Ronan
                                       -----------------------------------------
                                       Name:  T. Ronan
                                       Title: Director

                                     -11-
<PAGE>

                                    WELLS FARGO BANK (TEXAS), NATIONAL
                                    ASSOCIATION, as Lender

                                    By: /s/ Ann Rhoads
                                       -----------------------------------------
                                       Name:  Ann Rhoads
                                       Title: Vice President

                                    BANK OF SCOTLAND, as Lender

                                    By: /s/ Jack S. Dykes
                                       -----------------------------------------
                                       Name:  Jack S. Dykes
                                       Title: Executive Vice President

                                    HIBERNIA NATIONAL BANK, as Lender

                                    By: /s/ David R. Reid
                                       -----------------------------------------
                                       Name:  David R. Reid
                                       Title:  Senior Vice President

                                    BANK OF AMERICA, N.A., as Lender

                                    By: /s/ Irene C. Rummel
                                       -----------------------------------------
                                       Name:  Irene C. Rummel
                                       Title:  Vice President

                                    FIRST UNION NATIONAL BANK, Lender

                                    By: /s/ Paul N. Riddle
                                       -----------------------------------------
                                       Name:  Paul N. Riddle
                                       Title: Senior Vice President

                                     -12-
<PAGE>

                                    CREDIT AGRICOLE INDOSUEZ, as Lender

                                    By: /s/ Douglas A. Whiddon
                                       -----------------------------------------
                                       Name:  Douglas A. Whiddon
                                       Title: Senior Vice President
                                              Senior Relationship Manager

                                    By: /s/ Patrick Cocquerel
                                       -----------------------------------------
                                       Name:  Patrick Cocquerel
                                       Title: First Vice President, Managing
                                              Director, Head of Houston
                                              Representative Office

                                    UNION BANK OF CALIFORNIA, N.A., as Lender

                                    By: /s/ Dustin Gaspari
                                       -----------------------------------------
                                       Name:  Dustin Gaspari
                                       Title: Assistant Vice President

                                    By: /s/ John A. Clark
                                       -----------------------------------------
                                       Name:  John A. Clark
                                       Title: Vice President

                                    BANK ONE, TEXAS, N.A., as Lender

                                    By: /s/ Charles Kingwell-Smith
                                       -----------------------------------------
                                       Name:  Charles Kingwell-Smith
                                       Title: First Vice President

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