EXHIBIT 10.3FIRST AMENDMENT TO CREDIT AGREEMENT     THIS FIRST AMENDMENT TO CREDIT AGREEMENT (herein called the “Amendment”) made as ofOctober 24, 2007 by and among Targa Resources Partners LP, a Delaware limited partnership (the“Borrower”), Bank of America, N.A., as Administrative Agent (“AdministrativeAgent”), Collateral Agent, Swing Line Lender and L/C Issuer, and each Lender party hereto(collectively the “Lenders” and individually, a “Lender”).W I T N E S S E T H:     WHEREAS, the Borrower, Administrative Agent and Lenders entered into that certain CreditAgreement dated as of February 14, 2007 (as amended, supplemented, or restated to the date hereof,the “Original Agreement”), for the purpose and consideration therein expressed, wherebyLenders became obligated to make loans to the Borrower as therein provided; and     WHEREAS, the Borrower desires to amend the Original Agreement to increase the maximum amountof increases to the Aggregate Commitments that may be requested by the Borrower under Section 2.14;     NOW, THEREFORE, in consideration of the premises and the mutual covenants and agreementscontained herein and in the Original Agreement, in consideration of the loans which may hereafterbe made by Lenders to the Borrower, and for other good and valuable consideration, the receipt andsufficiency of which are hereby acknowledged, the parties hereto do hereby agree as follows:ARTICLE I.DEFINITIONS AND REFERENCES     Section 1.1. Terms Defined in the Original Agreement. Unless the context otherwiserequires or unless otherwise expressly defined herein, the terms defined in the Original Agreementshall have the same meanings whenever used in this Amendment.     Section 1.2. Other Defined Terms. Unless the context otherwise requires, thefollowing terms when used in this Amendment shall have the meanings assigned to them in thisSection 1.2.     “Amendment” means this First Amendment to Credit Agreement and the GuarantorRatification attached hereto.     “Credit Agreement” means the Original Agreement as amended hereby.FirstAmendment To Credit Agreement   1
 

 
ARTICLE II.AMENDMENTS TO ORIGINAL AGREEMENT     Section 2.1. Increase in Commitments. The Original Agreement is hereby amended toreplace the reference to “$250,000,000” in clause (ii) of Section 2.14(a) of the Original Agreementwith “$500,000,000”.ARTICLE III.CONDITIONS OF EFFECTIVENESS     Section 3.1. Effective Date. This Amendment shall become effective as of the datefirst above written when, and only when, (i) Administrative Agent shall have received, atAdministrative Agent’s office, a counterpart of this Amendment executed and delivered by theBorrower and Required Lenders and a counterpart of the Guarantor Ratification executed anddelivered by each Guarantor, (ii) the Borrower shall have requested an increase in the AggregateCommitments pursuant to Section 2.14 of the Credit Agreement in the amount of $250,000,000 and suchincrease in the Aggregate Commitments shall have become effective, and (iii) Administrative Agentshall have additionally received all of the following documents, each document (unless otherwiseindicated) being dated the date of receipt thereof by Administrative Agent, duly authorized,executed and delivered, and in form and substance satisfactory to Administrative Agent:     (a) a favorable opinion of Bracewell & Giuliani LLP, counsel to the Loan Parties,addressed to the Administrative Agent and each Lender, as to such matters as theAdministrative Agent may reasonably request;     (b) a certificate of the Secretary of each Loan Party certifying that none of theresolutions or other action, incumbency certificates and/or other certificates ofResponsible Officers of each Loan Party as the Administrative Agent has previously requiredevidencing the identity, authority and capacity of each Responsible Officer thereofauthorized to act as a Responsible Officer in connection with this Amendment and the otherLoan Documents to which such Loan Party is a party have been amended since they weredelivered,     (c) such documents and certifications as the Administrative Agent may reasonablyrequire to evidence that each Loan Party is duly organized or formed, and that each LoanParty is validly existing, in good standing and qualified to engage in business in eachjurisdiction where its ownership, lease or operation of properties or the conduct of itsbusiness requires such qualification; and that the execution and delivery of this Amendmenthas been duly authorized; and     (d) the Borrower shall have paid all fees required to be paid to Administrative Agentpursuant to any Loan Documents and all other fees and reimbursements to be paid pursuant toany Loan Documents, including fees and disbursements of Administrative Agent’s attorneys.FirstAmendment To Credit Agreement   2
 

 
ARTICLE IV.REPRESENTATIONS AND WARRANTIES     Section 4.1. Representations and Warranties of the Borrower. In order to induce eachLender to enter into this Amendment, the Borrower represents and warrants to each Lender that,after giving effect to this Amendment (and including for purposes of all references to the LoanDocuments, and the Credit Agreement, (i) the representations and warranties of the Borrower andeach Loan Party contained in Article V of the Original Agreement or any other Loan Document, orwhich are contained in any document furnished at any time under or in connection therewith, aretrue and correct in all material respects on and as of the time of the effectiveness hereof, exceptto the extent such representations and warranties specifically refer to an earlier date, in whichcase they are true and correct as of such earlier date, and for purposes of this Section4.1, the representations and warranties contained in subsections (a) and (b) ofSection 5.05 of the Credit Agreement shall be deemed to refer, to the most recentstatements furnished pursuant to clauses (a) and (b), respectively, of Section 6.01 of theCredit Agreement, and (ii) no Default exists.ARTICLE V.MISCELLANEOUS     Section 5.1. Ratification of Agreements. The Original Agreement as hereby amended ishereby ratified and confirmed in all respects. The other Loan Documents, as they may be amended oraffected by this Amendment, are hereby ratified and confirmed in all respects. Any reference to theCredit Agreement in any Loan Document shall be deemed to be a reference to the Original Agreementas hereby amended. The execution, delivery and effectiveness of this Amendment shall not, except asexpressly provided herein or therein, operate as a waiver of any right, power, or remedy ofAdministrative Agent, Swing Line Lender, L/C Issuer, or Lenders under the Credit Agreement or anyother Loan Document nor constitute a waiver of any provision of the Credit Agreement or any otherLoan Document.     Section 5.2. Survival of Agreements. All representations, warranties, covenants, andagreements of the Borrower herein shall survive the execution and delivery of this Amendment andthe performance thereof, and shall further survive until all of the Obligations are paid in full.All statements and agreements contained in this Amendment or any certificate or instrumentdelivered by any Loan Party hereunder or thereunder to Administrative Agent, L/C Issuer, Swing LineLender, or any Lender shall be deemed to constitute representations and warranties by, and/oragreements and covenants of the Borrower and such Loan Party under this Amendment and under theCredit Agreement.     Section 5.3. Loan Documents. This Amendment is a Loan Document, and all provisions inthe Credit Agreement pertaining to Loan Documents apply hereto and thereto.     Section 5.4. Governing Law. This Amendment shall be governed by and construed inaccordance with, the law of the State of New York.FirstAmendment To Credit Agreement   3
 

 
     Section 5.5. Counterparts; Fax. This Amendment may be separately executed incounterparts and by the different parties hereto in separate counterparts, each of which when soexecuted shall be deemed to constitute one and the same agreement. This Amendment may be validlyexecuted by facsimile or other electronic transmission.     **THIS AMENDMENT AND THE OTHER LOAN DOCUMENTS REPRESENT THE FINAL AGREEMENT BETWEEN THE PARTIESAND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENTS OFTHE PARTIES. THERE ARE NO UNWRITTEN ORAL AGREEMENTS OF THE PARTIES.**[*The remainder of this page has been intentionally left blank.*]FirstAmendment To Credit Agreement   4
 

 
     IN WITNESS WHEREOF, this Amendment is executed as of the date first above written.           **TARGA RESOURCES PARTNERS LP**           By: Targa Resources GP LLC, its sole generalpartner               By: /s/ Howard M. Tate          Howard M. Tate        Vice President — Finance andTreasurer  SIGNATURE PAGE TO FIRST AMENDMENT TO CREDIT AGREEMENT (TARGA RESOURCES PARTNERS LP)
 

 
         **BANK OF AMERICA, N.A.**, as Administrative Agent           By: /s/ Todd Mac Neill        Name: Todd Mac Neill      Title: Vice President  SIGNATURE PAGE TO FIRST AMENDMENT TO CREDIT AGREEMENT (TARGA RESOURCES PARTNERS LP)
 

 
         **BANK OF AMERICA, N.A.**, as a Lender,L/C Issuer and Swing Line Lender           By: /s/ Christopher Smith       Name: Christopher Smith      Title: Senior Vice President  SIGNATURE PAGE TO FIRST AMENDMENT TO CREDIT AGREEMENT (TARGA RESOURCES PARTNERS LP)
 

 
         **WACHOVIA BANK, NATIONAL ASSOCIATION**, as SyndicationAgent and as a Lender           By: /s/ Leanne S. Phillips       Name: Leanne S. Phillips      Title: Director  SIGNATURE PAGE TO FIRST AMENDMENT TO CREDIT AGREEMENT (TARGA RESOURCES PARTNERS LP)
 

 
         **MERRILL LYNCH CAPITAL, A DIVISION OF MERRILL LYNCHBUSINESS FINANCIAL SERVICES INC.**, asCo-Documentation Agent and as a Lender           By: /s/ Gregory B. Hanson       Name: Gregory B. Hanson      Title: Vice President  SIGNATURE PAGE TO FIRST AMENDMENT TO CREDIT AGREEMENT (TARGA RESOURCES PARTNERS LP)
 

 
         **ROYAL BANK OF CANADA**, asCo-Documentation Agent and as a Lender           By: /s/ David A. McCluskey       Name: David A. McCluskey      Title: Authorized Signatory  SIGNATURE PAGE TO FIRST AMENDMENT TO CREDIT AGREEMENT (TARGA RESOURCES PARTNERS LP)
 

 
         **THE ROYAL BANK OF SCOTLAND PLC**, as