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                                                                  Exhibit 10(xi)

                       CONFIDENTIAL TREATMENT REQUESTED
                       --------------------------------

     Confidential Portions of This Agreement Which Have Been Redacted Are Marked
With Brackets ("[***]"). The Omitted Material Has Been Filed Separately With The
United States Securities and Exchange Commission.


                      AMENDMENT NO. 1 TO CREDIT AGREEMENT
                      -----------------------------------


     THIS AMENDMENT NO. 1 TO CREDIT AGREEMENT (this "Amendment") dated as of May
                                                     ---------
22, 2001, by and among NATIONAL DATA CORPORATION, a Delaware corporation, as
Borrower, the banks and other financial institutions listed on the signature
pages hereof, as Lenders, and BANK ONE, NA, a national banking association
having its principal office in Chicago, Illinois, as Administrative Agent, Swing
Line Lender and LC Issuer.

                              W I T N E S S E T H:
                              - - - - - - - - - -

     WHEREAS, Borrower, the Lenders, and the Administrative Agent are parties to
a certain Credit Agreement dated as of January 31, 2001 (the "Credit Agreement";
capitalized terms used in this Amendment without definition that are defined in
the Credit Agreement shall have the meanings in this Amendment as specified for
such capitalized terms in the Credit Agreement);

     WHEREAS, Borrower and Lenders have agreed to amend the Credit Agreement in
certain respects as set forth in this Amendment;

     NOW, THEREFORE, for and in consideration of the mutual covenants contained
herein and other valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, the parties hereto, intending to be legally bound,
agree as follows:

     SECTION 1.  Amendments to Credit Agreement.  Subject to the satisfaction of
                 ------------------------------
the conditions precedent set forth in Section 2 hereof, and effective as of the
Effective Date (as hereinafter defined), the Credit Agreement is hereby amended
as follows:

     (a) Section 1.1 of the Credit Agreement is hereby amended by adding the new
defined terms "Initial MedUnite Assets", "Initial MedUnite Transaction",
               -----------------------    ----------------------------
"MedUnite",  "MedUnite Transactions" and "Subsequent MedUnite Transactions" and
---------     ---------------------       --------------------------------
accompanying definitions, in appropriate alphabetical order, as follows:

          "Initial MedUnite Assets" means those assets to be transferred to
           -----------------------
     MedUnite pursuant to the Initial MedUnite Transaction, such assets to be
     comprised of tangible assets having an aggregate book value of
     approximately [***] (in each case as reflected on the Borrower's March 31,
     2001 pro forma financial statements furnished to the Lenders), all of which
     assets are used solely in connection with the Borrower's physician real-
     time and batch clearinghouse network business based in Norcross, Georgia,
     and include the Norcross computer network data center, and contracts with
     payers, providers and EDI clearinghouses.
<PAGE>
                                        [***] - CONFIDENTIAL TREATMENT REQUESTED

          "Initial MedUnite Transaction" means the transfer and assignment by
           ----------------------------
     Borrower to MedUnite of the Initial MedUnite Assets, together with
     contributed liabilities of the Borrower of approximately [***] (as
     reflected on the Borrower's March 31, 2001 pro forma financial statements
     furnished to the Lenders) to be assumed by MedUnite, in exchange for common
     stock of MedUnite representing 17.9% of the total equity of MedUnite.

          "MedUnite" means MedUnite Inc., a Delaware corporation.
           --------

          "MedUnite Transactions" means, collectively, the Initial MedUnite
           ---------------------
     Transaction and any Subsequent MedUnite Transactions.

          "Subsequent MedUnite Transactions" means one or more cash Investments
           --------------------------------
     made by the Borrower in MedUnite, subsequent to the Initial MedUnite
     Transaction, in exchange for common stock of MedUnite in an aggregate
     amount for all such cash Investments not to exceed [***].

     (b) Section 6.6(a) of the Credit Agreement is hereby amended by deleting
clauses (iii) and (vi) of said Section 6.6(a) in their entirety and substituting
in lieu thereof the following clauses (iii) and (vi):

          (iii)  Investment in MedUnite pursuant to the MedUnite Transactions
     and any other capital contributions of assets expressly permitted by
     Section 6.9;

     . . . .

          (vi)   Other Investments not described in the preceding clauses (i)
     through (iv) made in an aggregate amount during any period of four
     consecutive Fiscal Quarters (excluding Fiscal Quarters ending on or before
     November 30, 2000) not to exceed $10,000,000 provided that no such
     Investments pursuant to this clause (vi) shall be made in respect of
     MedUnite.

     (c) Section 6.9 of the Credit Agreement is hereby amended by deleting
clause (D) of said Section 6.9 in its entirety and substituting in lieu thereof
the following clause (D):

          (D)    so long as no Default or Event of Default shall be in existence
     either immediately prior to or following any asset disposition, the
     Borrower and its respective Subsidiaries may effect (x) the MedUnite
     Transactions, and (y) any other Asset Sale so long as the value of the
     assets sold (measured at the higher of book value or the total sale price
     for such assets) pursuant to all such other Asset Sales during any period
     of four (4) consecutive Fiscal Quarters (excluding Fiscal Quarters ending
     on or before November 30, 2000) does not exceed $25,000,000, and

          SECTION 2.  Conditions to Effectiveness of Amendment.  This Amendment
                      ----------------------------------------
shall become effective on the first day when the Administrative Agent shall have
received all of the following:  (i) counterparts of this Amendment as executed
on behalf of Borrower and the Lenders, together with the Acknowledgment and
Agreement of Subsidiary Guarantors as executed on behalf of the Subsidiary
Guarantors, and (ii) payment to the Administrative Agent,

                                      -2-
<PAGE>

for the account of each Lender, of an amendment fee equal to one-tenth of one
percent (0.10%) of each such Lender's Commitment. Such date shall be the
"Amendment No. 1 Effective Date" for purposes of this Amendment.
--------------------------------

          SECTION 3.  Status of Obligations.  Borrower hereby confirms and
                      ---------------------
agrees that all Loans and all other Obligations outstanding under the Credit
Agreement and the other Loan Documents as of the date hereof were duly and
validly created and incurred by Borrower thereunder, that all such outstanding
amounts are owed in accordance with the terms of the Credit Agreement and other
Loan Documents, and that there are no rights of offset, defense, counterclaim,
claim or objection in favor of Borrower arising out of or with respect to any of
the Loans or other Obligations of Borrower to the Administrative Agent or the
Lenders, and any such rights of offset, defense, counterclaim, claims or
objections have been and are hereby waived and released by Borrower.

          SECTION 4.  Representations and Warranties of Borrower.  Borrower,
                      ------------------------------------------
without limiting the representations and warranties provided in the Credit
Agreement, represents and warrants to the Lenders and the Administrative Agent
as follows:

          4.1  The execution, delivery and performance by Borrower of this
Amendment are within Borrower's corporate powers, have been duly authorized by
all necessary corporate action (including any necessary shareholder action) and
do not and will not (a) violate any provision of any law, rule or regulation,
any judgment, order or ruling of any court or governmental agency, the
certificate of incorporation or by-laws of Borrower, or any indenture, agreement
or other instrument to which Borrower is a party or by which Borrower or any of
its properties is bound or (b) be in conflict with, result in a breach of, or
constitute with notice or lapse of time or both a default under any such
indenture, agreement or other instrument.

          4.2  This Amendment constitutes the legal, valid and binding
obligation of Borrower, enforceable against Borrower in accordance with its
terms.

          4.3  After giving effect to this Amendment, no Default or Event of
Default has occurred and is continuing.

          4.4  The representations and warranties of Borrower contained in the
Credit Agreement are true and accurate on and as of the date of this Amendment,
except for changes expressly permitted under the terms of the Credit Agreement
and except to the extent that such representations and warranties relate solely
to an earlier date (in which case such representations and warranties were true
and accurate as of such earlier date).

          4.5  Since February 28, 2001, there have been no events, acts,
conditions or occurrences of whatever nature, singly or in the aggregate, which
have had, or could reasonably be expected to have, a Material Adverse Effect.

          SECTION 5.  Survival.  Each of the foregoing representations and
                      --------
warranties shall be made at and as of the date of this Amendment and shall be
deemed to have been made as of the Amendment No. 1 Effective Date.  Each of the
foregoing representations and warranties shall constitute a representation and
warranty of Borrower under the Credit Agreement, and it shall be an Event of
Default if any such representation and warranty shall prove to have been

                                      -3-
<PAGE>

incorrect or false in any material respect at the time when made or deemed to
have been made.  Each of the foregoing representations and warranties shall
survive and not be waived by the execution and delivery of this Amendment or any
investigation by the Lenders or the Administrative Agent.

          SECTION 6.  Ratification of Credit Agreement and Loan Documents.
                      ---------------------------------------------------
Except as expressly amended herein, all terms, covenants and conditions of the
Credit Agreement and the other Loan Documents shall remain in full force and
effect, and the parties hereto do expressly ratify and confirm the Credit
Agreement (as amended herein) and the other Loan Documents.  All future
references to the Credit Agreement shall be deemed to refer to the Credit
Agreement as amended hereby.

          SECTION 7.  Indemnity.   In consideration of the amendments agreed to
                      ----------
by the Lenders pursuant to this Amendment, Borrower hereby indemnifies the
Administrative Agent, and each Lender, and their respective officers, partners,
directors, employees, representatives and agents from, and hold each of them
harmless against, any and all costs, losses, liabilities, claims, damages or
expenses incurred by any of them (whether or not any of them is designated a
party thereto) (an "Indemnitee") arising out of or by reason of any
investigation, litigation or other proceeding related to this Amendment, the
Credit Agreement or any other Loan Documents or any actual or proposed use of
the proceeds of any of the Loans, including, without limitation, the reasonable
fees and disbursements of counsel incurred in  connection with any such
investigation, litigation or other proceeding; provided, however, Borrower shall
not be obligated to indemnify any Indemnitee for any of the foregoing arising
out of such Indemnitee's gross negligence or willful misconduct.

          SECTION 8.  No Waiver, Etc.  Borrower hereby agrees that nothing
                      ---------------
herein shall constitute a waiver by the Lenders of any Default or Event of
Default, whether known or unknown, which may exist under the Credit Agreement.
Borrower hereby further agrees that no action, inaction or agreement by the
Lenders, including without limitation, any indulgence, waiver, consent or
agreement altering the provisions of the Credit Agreement which may have
occurred with respect to the non-performance of any obligation under the terms
of the Credit Agreement or any portion thereof, or any other matter relating to
the Credit Agreement, shall require or imply any future indulgence, waiver, or
agreement by the Lenders.

          SECTION 9.  Binding Nature.  This Amendment shall be binding upon and
                      --------------
inure to the benefit of the parties hereto, their respective successors,
successors-in-titles, and assigns.

          SECTION 10. Costs and Expenses.  Borrower shall be responsible for
                      ------------------
the costs and expenses of the Administrative Agent in connection with the
preparation, execution and delivery of this Amendment and the other instruments
and documents to be delivered hereunder, including, without limitation, the fees
and out-of-pocket expenses of counsel for the Administrative Agent with respect
thereto.

          SECTION 11. GOVERNING LAW.  THIS AMENDMENT SHALL BE GOVERNED BY, AND
                      -------------
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF GEORGIA.

                                      -4-
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          SECTION 12. Entire Understanding.  This Amendment sets forth the
                      --------------------
entire understanding of the parties with respect to the matters set forth
herein, and shall supersede any prior negotiations or agreements, whether
written or oral, with respect thereto.

          SECTION 13. Counterparts.  This Amendment may be executed in any
                      ------------
number of counterparts and by the different parties hereto in separate
counterparts and may be delivered by telecopier.  Each counterpart so executed
and delivered shall be  deemed an original and all of which taken together shall
constitute but one and the same instrument.

                                      -5-
<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
be duly executed and delivered by their duly authorized officers as of the day
and year first above written.

                              NATIONAL DATA CORPORATION


                              By: /s/ Patricia A. Wilson
                                  ----------------------------------
                                  Name:  Patricia A. Wilson
                                  Title: General Counsel

                                      -6-
<PAGE>

                              BANK ONE, NA,
                              as Administrative Agent, Lender, LC Issuer
                              and Swing Line Lender


                              By:  /s/ Jennifer Schmoll
                                   --------------------------------
                                   Name:  Jennifer Schmoll
                                   Title: Customer Service Officer

                      [Signature page to Amendment No. 1]

<PAGE>

                              SUNTRUST BANK,
                              as Lender


                              By:  /s/ Robert Massenburg
                                   -------------------------------
                                   Name:  Robert Massenburg
                                   Title: Director

                      [Signature page to Amendment No. 1]

<PAGE>

                              WACHOVIA BANK, N.A.
                              as Lender


                              By:  /s/ Karen H. McClain
                                   -------------------------------
                                   Name:  Karen H. McClain
                                   Title: Senior Vice President

                      [Signature page to Amendment No. 1]

<PAGE>

             ACKNOWLEDGMENT AND AGREEMENT OF SUBSIDIARY GUARANTORS
             -----------------------------------------------------


          Reference is hereby made to the within and foregoing Amendment No. 1
to Credit Agreement, dated as of May 22, 2001, by and among NATIONAL DATA
CORPORATION, a Delaware corporation ("Borrower"), BANK ONE, NA, a national
banking association, as Administrative Agent, Lender, Swing Line Lender and LC
Issuer, SUNTRUST BANK, a Georgia banking corporation, as Lender, and WACHOVIA
BANK, N.A., a national banking association, as Lender ("Amendment No. 1";
capitalized terms used herein that are defined in Amendment No. 1 or in the
"Credit Agreement" as defined in Amendment No. 1 being used herein with the
respective meanings assigned to such capitalized terms in Amendment No. 1 or the
Credit Agreement, as the case may be).  Each of the undersigned, which is a
Subsidiary Guarantor under the terms of the Subsidiary Guarantee as provided in
the Credit Agreement, hereby acknowledges and agrees that (i) the undersigned
has consented to the foregoing Amendment No. 1, (ii) the Subsidiary Guarantee
and the other Loan Documents to which each of the undersigned is a party shall
remain in full force and effect on and after the date hereof, and (iii)  each of
the undersigned hereby reaffirms and restates its obligations and liabilities
under the Subsidiary Guarantee and the other Loan Documents to which each of
the undersigned is a party after giving effect to Amendment No. 1.

     This Acknowledgment and Agreement of Subsidiary Guarantors made and
delivered as of May 22, 2001.

                         GUARANTORS:
                         ----------

                         NDC HEALTH INFORMATION SERVICES
                         (ARIZONA) INC., as a Subsidiary Guarantor


                         By:  ________________________________
                              Name:
                              Title:


                         SOURCE INFORMATICS INC.,
                         as a Subsidiary Guarantor


                         By:  ________________________________
                              Name:
                              Title:

                                      -10-
<PAGE>

                         THE COMPUTER PLACE, INC.,
                         as a Subsidiary Guarantor


                         By:  ________________________________
                              Name:
                              Title:

                                      -11-