<PAGE>   1
                                                                  EXHIBIT 10.21

                               FIRST AMENDMENT TO
                                 LOAN AGREEMENT


         This FIRST AMENDMENT TO LOAN AGREEMENT (this "Amendment"), dated as of
March 31, 1999, is made and entered into between BANK ONE, TEXAS, NATIONAL
ASSOCIATION ("Bank"), and MEDICALCONTROL, INC., a Delaware corporation
("Borrower").

                                    RECITALS

         A. Bank and Borrower entered into that certain Loan Agreement, dated
September 25, 1998 (the "Loan Agreement").

         B. Bank and Borrower desire to amend the Loan Agreement as herein set
forth.

         NOW, THEREFORE, in consideration of the premises herein contained and
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the parties hereto agree as follows:

                                   ARTICLE I
                                  Definitions

         Section 1.01. Definitions. Capitalized terms used in this Amendment,
to the extent not otherwise defined herein, shall have the same definitions
assigned to such terms in the Loan Agreement, as amended hereby.

                                   ARTICLE II
                        Amendments to the Loan Agreement

         Section 2.01. Amendment to Paragraph 1, Credit Facilities. From and
after the date hereof, paragraph 1 of the Loan Agreement is deleted in its
entirety and in place thereof shall be a new paragraph 1 which shall read as
follows:

                  "1. CREDIT FACILITIES. Subject to the terms and conditions
         set forth in this Loan Agreement and the other agreements, instruments
         and documents evidencing, securing, governing, guaranteeing and/or
         pertaining to the Loans, as hereinafter defined (collectively,
         together with the Loan Agreement, referred to hereinafter as the "LOAN
         DOCUMENTS"), Bank hereby agrees to lend to Borrower (a) the amount of
         $1,600,000 (the "TERM LOAN") in a single advance on the date hereof,
         and (b) (the "LINE OF CREDIT"), on a revolving basis from time to time
         during the period commencing on the date hereof and continuing through
         the maturity date of the promissory note evidencing this credit
         facility from time to time, such amounts as Borrower may request
         hereunder; provided, however, the total principal amount outstanding
         at any time under the Line of Credit shall not





<PAGE>   2




         exceed $400,000 (the Line of Credit and the Term Loan are collectively
         referred to herein as the "CREDIT FACILITIES"). Subject to the terms
         and conditions hereof, Borrower may borrow, repay and reborrow
         hereunder. The sums hereafter advanced under the Loans shall be used
         for general working capital purposes.

         All advances under the Credit Facilities shall be collectively called
         the "Loans." Bank reserves the right to require Borrower to give Bank
         not less than one (1) business day's prior notice of each requested
         advance under the Line of Credit, specifying (i) the aggregate amount
         of such requested advance, (ii) the requested date of such advance,
         and (iii) the purpose for such advance, with such advances to be
         requested in a form satisfactory to Bank. In addition, Borrower shall
         pay to Bank during the term of the Line of Credit an unused fee equal
         to .25% per annum quarterly in arrears, of an amount equal to $400,000
         less the average outstanding balance under the Line of Credit."

         Section 2.02. Amendment to Paragraph 5, Guarantors. From and after the
date hereof, paragraph 5 of the Loan Agreement is deleted in its entirety and
in place thereof shall be a new paragraph 5 which shall read as follows:

                  "5. GUARANTORS. As a condition precedent to the Bank's
         obligation to make the Loans to Borrower, Borrower agrees to cause
         MedicalControl Network Solutions, Inc., Diversified Group
         Administrators, Inc., ppoONE.com, inc. f/k/a PPO Management Solutions,
         Inc., Medical Control Holdings, Inc. and Business Health Companies,
         Inc. (whether one or more, the "GUARANTORS") to each execute and
         deliver to Bank contemporaneously herewith a guaranty agreement, in
         form and substance satisfactory to Bank."

         Section 2.03. Amendment to Paragraph 8, Affirmative Covenants. From
and after the date hereof, paragraph 8 of the Loan Agreement is amended by the
addition of new subparagraphs 8 (l) and (m) which shall read as follows:

                  "(l) Secondary Equity Offering. In the event that Borrower
         makes any additional equity offering (excluding an offer of preferred
         stock referred to in subparagraph (m) below), all proceeds from such
         offering shall be applied to the Loans in such order as Bank may
         require.

                  (m) Preferred Stock Offering. In the event that Borrower
         makes any preferred stock offering, the first $500,000 in proceeds
         from such offering shall be applied to the Loans in such order as Bank
         may require."

         Section 2.04. Amendment to Paragraph 9, Negative Covenants. From and
after the date hereof, paragraph 9 of the Loan Agreement is amended by the
addition of a new subparagraph 9 (j) which shall read as follows:




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                  "(j) Acquisitions. Acquire (i) all or substantially all of
         the assets of any Person, (ii) the assets constituting the business or
         a division or operating unit of any Person, or (iii) more than ten
         percent (10%) of the capital stock of any Person. As used here,
         'Person' means an individual, corporation, limited liability company,
         partnership, joint venture, association, trust unincorporated
         organization or a government or any agency or political subdivision
         thereof."

         Section 2.05. Amendment to Paragraph 10, Financial Covenants. From and
after the date hereof, subparagraphs 10 (b), (c), (d), (e) and (f) of the Loan
Agreement are deleted in their entirety and in place thereof shall be new
subparagraphs 10 (b), (c), (d), (e) and (f) which shall read as follows:

                  "(b) Net Worth. Borrower will maintain, at all times, its
         total assets less its total liabilities at not less than $7,700,000
         plus one hundred percent (100%) of additional equity contributions.

                  (c) Debt Service. Borrower will maintain, as of the last day
         of each fiscal quarter commencing September 30, 1999, a ratio of (i)
         consolidated net income after taxes plus interest expense,
         depreciation and amortization, less any capital expenditures, to (ii)
         total interest expense and current maturities of long term debt and
         long term capital leases (excluding the deferred portion of the
         Subordinated Debt), of not less than 1.25 to 1.0. The foregoing
         covenant shall be measured from January 1, 1999 on a year to date
         basis until December 31, 1999, and on a rolling twelve month basis
         thereafter

                  (d) Funded Debt to Cash Flow. Borrower will maintain, as of
         each fiscal quarter end set forth below, a ratio of all outstanding
         Senior Funded Debt of Borrower at the such quarter end, to (ii)
         consolidated net income after taxes plus interest expense,
         depreciation and amortization for the period then ending, of not more
         than the ratio set forth opposite such quarter end below.
<TABLE>
<CAPTION>

         Quarter End:                                         Ratio:
         -----------                                          -----
<S>                                                           <C>
         June 30, 1999                                        2.25 to 1.0
         September 30, 1999 and each
                  fiscal quarter end thereafter               2.00 to 1.0
</TABLE>

         The foregoing covenant shall be measured from January 1, 1999 on a
         year to date basis until December 31, 1999, and on a rolling twelve
         month basis thereafter. As used herein, the term "Senior Funded Debt"
         means, as of any date, the sum of the following (without duplication):
         (i) the aggregate of all indebtedness for money borrowed of Borrower
         as of such date, other than current liabilities, (ii) all indebtedness
         which would be classified as "funded indebtedness" or "long-term
         indebtedness" (or other similar classification) on a consolidated
         balance sheet of Borrower prepared as of such date in accordance with
         generally accepted accounting principles, (iii) the aggregate of all
         indebtedness of



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<PAGE>   4




         Borrower outstanding under any revolving credit or similar agreement
         providing for borrowing (and renewals and extensions thereof) over a
         period of more than one year, notwithstanding the fact that any such
         indebtedness is created within one year of the expiration of such
         agreement, and (iv) the amount of all capitalized lease obligations of
         Borrower booked in accordance with generally accepted accounting
         principles, minus (v) the Subordinated Debt.

                  (e) Capital Expenditures. Borrower will not make capital
         expenditures in excess of $150,000 during any fiscal year.

                  (f) Current Ratio. As of the end of each fiscal quarter of
         Borrower, a ratio of (a) current assets, (including prepaid expenses)
         to (b) current liabilities of not less than 1.0 to 1.0"

         Section 2.06. Amendment to Paragraph 10, Financial Covenants. From and
after the date hereof, paragraph 10 of the Loan Agreement is amended by the
addition of a new subparagraph 10 (h) which shall read as follows:

                  "(h) Minimum Earnings. As of each date set forth below,
         Borrower shall have earnings plus depreciation, amortization, and
         interest expense of not less than the amount set forth opposite such
         date below measured from January 1, 1999, on a year to date basis:
<TABLE>
<CAPTION>
                 Date:                      Required EBIDA:
                 -----                      ---------------
<S>                                         <C>      
            March 31, 1999                    $  20,000
            April 30, 1999                    $  60,000
            May 31, 1999                      $100,000
            June 30, 1999                     $160,000"
</TABLE>

         Section 2.07. Amendment to Paragraph 11, Reporting Requirements. From
and after the date hereof, paragraph 11 (a) of the Loan Agreement is deleted in
its entirety and in place thereof shall be a new paragraph 11 (a) which shall
read as follows:

                  "(a) Interim Financial Statements. As soon as available, and
         in any event within thirty (30) days after the end of each month, a
         consolidated and consolidating balance sheet and income statement of
         Borrower as of the end of such month, all in form and substance and in
         reasonable detail satisfactory to Bank and duly certified (subject to
         year end review adjustments) by the President and/or Chief Financial
         Officer of Borrower (i) as being true and correct in all material
         aspects to the best of his or her knowledge, (ii) as having been
         prepared in accordance with generally accepted accounting principles,
         consistently applied, and (iii) as being consistent in all material
         respects with the



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         information submitted by Borrower to the U.S. Securities and Exchange 
         Commission pursuant to its most recent Quarterly Report on Form 10QSB."

         Section 2.08. Amendment to Paragraph 12, Events of Default. From and
after the date hereof, paragraph 12 of the Loan Agreement is amended by the
addition of new subparagraphs 12 (j) and (k) which shall read as follows:

                  "(j) The failure of Borrower, on or before 5:00 p.m. on April
         15, 1999, (i) to receive an equity contribution from J. Ward Hunt in
         the amount of at least $70,000, and (ii) to deliver to, and pledge to,
         Bank the sum of $70,000 as additional as collateral for the Loans.

                  (k) The failure of Borrower, on or before 5:00 p.m. on April
         2, 1999, to deliver to Bank (i) original, executed counterparts of
         that certain First Amendment to Loan Agreement between the Bank and
         Borrower dated as of March 31, 1999, and all other Loan Documents to
         be executed in connection therewith, and (ii) original corporate
         resolutions, with secretary's certificates attached, for Borrower and
         for each Guarantor who executed any of such Loan Documents, all in
         form and substance acceptable to Bank.

                  (l)  The failure of Borrower, within three (3) days of its
         receipt thereof, to deliver to Bank as a prepayment on the Term Loan
         all of Borrower's federal income tax refund in the approximate amount
         of $350,000 expected in June 1999."

                                  ARTICLE III
                                Covenant Waivers

         Section 3.01. Waiver of Certain Financial Covenants. Bank hereby
waives Borrower's compliance with subparagraphs 10 (c), (d), and (f) for the
fiscal quarters of Borrower ending September 30, 1998 and December 31, 1998.
Such waiver is expressly limited as aforesaid and shall not constitute a waiver
of Borrower's compliance with such covenants for any other past or future
period or of any other breach of the Loan Agreement or any other Loan Document.

                                   ARTICLE IV
                              Conditions Precedent

         Section 4.01. Conditions Precedent. The effectiveness of this
Amendment is subject to the satisfaction of the following conditions precedent,
unless specifically waived in writing by Bank:

                  (a) Borrower shall have executed and delivered to Bank this
         Amendment, and such other Loan Documents as Bank may require.

                  (b) Medical Control Holdings, Inc. shall have executed and
         delivered to Bank (i) a Pledge Agreement pledging to Bank all of the
         issued and outstanding capital stock of Medical Control Network
         Solutions, Inc., Diversified Group Administrators, Inc. and


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         ppoONE.com, inc., and (ii) original stock certificates evidencing all
         such stock either endorsed in blank or accompanied by stock powers
         endorsed in blank.

                  (c) Medical Control Network Solutions, Inc. shall have
         executed and delivered to Bank (i) a Pledge Agreement pledging to Bank
         all of the issued and outstanding capital stock of Business Health
         Companies, Inc., and (ii) original stock certificates evidencing all
         such stock (save and except Certificate No. 4 that evidences 140
         shares pledged to Ralph T. Smith, Douglas L. Elden and Donald R.
         Huntington) either endorsed in blank or accompanied by stock powers
         endorsed in blank.

                  (d) Borrower shall have delivered to Bank the sum of $200,000
         as a prepayment of the Term Loan.

                  (e) Borrower shall have provided to Bank reasonable evidence
         that J. Ward Hunt has contributed to Borrower not less than $70,000 in
         equity within the last fifteen days, which amount shall have been used
         to make payments on the promissory notes evidencing the Subordinated
         Debt held by Ralph T. Smith, Douglas L. Elden and Donald R.
         Huntington.

                  (f) Borrower shall have delivered to Bank copies of all
         original documents and instruments executed in connection with the
         Subordinated Debt held by Ralph T. Smith, Douglas L. Elden and Donald
         R. Huntington including, without limitation, the promissory notes
         evidencing such debt.

                  (g) Each Guarantor (other than Business Health Companies,
         Inc. and Medical Control Holdings, Inc.) shall have executed and
         delivered this Amendment.

                  (h) Each of Business Health Companies, Inc. and Medical
         Control Holdings, Inc. shall have executed and delivered to Bank an
         Unlimited Guaranty in form and substance acceptable to Bank and
         Business Health Companies, Inc. and the other Guarantors (other than
         Medical Control Holdings, Inc.) shall have executed and delivered to
         Bank an Addendum to Security Agreement to add Business Health
         Companies, Inc. as a party to the Security Agreement dated September
         25, 1998 executed by the Guarantors (other than Medical Control
         Holdings, Inc.).

                  (i) The representations and warranties contained herein and
         in all other Loan Documents shall be true and correct as of the date
         hereof as if made on the date hereof.

                  (j) Except as waived in writing by Bank, no Event of Default
         shall have occurred and be continuing and no event or condition shall
         have occurred that with the giving of notice or lapse of time or both
         would be an Event of Default.



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                  (k) All corporate proceedings taken in connection with the
         transactions contemplated by this Amendment and all documents,
         instruments and other legal matters incident thereto shall be
         satisfactory to Bank and its legal counsel in their sole discretion.

                                   ARTICLE V
                 Ratifications, Representations, and Warranties

         Section 5.01. Ratifications by Borrower . The terms and provisions set
forth in this Amendment shall modify and supersede all inconsistent terms and
provisions set forth in the Loan Agreement and, except as expressly modified
and superseded by this Amendment, the terms and provisions of the Loan
Agreement are ratified and confirmed and shall continue in full force and
effect. The Loan Agreement as amended by this Amendment shall continue to be
legal, valid, binding and enforceable in accordance with its terms.

         Section 5.02. Renewal and Extension of Security Interests and Liens.
Each of Borrower and each Guarantor hereby renews and affirms the liens and
security interests created and granted in the Loan Documents. Each of Borrower
and each Guarantor agrees that this Amendment shall in no manner affect or
impair the liens and security interests securing the Indebtedness, and that
such liens and security interests shall not in any manner be waived, the
purposes of this Amendment being to modify the Loan Agreement as herein
provided, and to carry forward all liens and security interest securing same,
which are acknowledged by Borrower and each Guarantor to be valid and
subsisting.

         Section 5.03. Representations and Warranties. Borrower represents and
warrants to Bank as follows: (i) the execution, delivery and performance of
this Amendment and any and all Loan Documents executed and/or delivered by
Borrower in connection herewith have been authorized by all requisite corporate
action on the part of Borrower and will not violate the articles or bylaws of
Borrower or any agreement to which Borrower is a party; (ii) the
representations and warranties contained in the Loan Agreement as amended
hereby and in each of the other Loan Documents are true and correct on and as
of the date hereof as though made on and as of the date hereof; (iii) no Event
of Default under the Loan Agreement has occurred and is continuing; and (iv)
Borrower is in full compliance with all covenants and agreements contained in
the Loan Agreement, as amended hereby.

                                   ARTICLE VI
                                 Miscellaneous

         Section 6.01. Survival of Representations and Warranties. All
representations and warranties made in the Loan Agreement or any other Loan
Document including, without limitation, any Loan Documents furnished in
connection with this Amendment, shall survive the execution and delivery of
this Amendment and the other Loan Documents, and no investigation



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by Bank or any closing shall affect such representations and warranties or the
right of Bank to rely thereon.

         Section 6.02. Reference to Loan Agreement. Each of the Loan Documents
and the Loan Agreement and any and all other agreements, documents or
instruments now or hereafter executed and delivered pursuant to the terms
hereof or pursuant to the terms of the Loan Agreement as amended hereby, are
hereby amended so that any reference in such Loan Documents to the Loan
Agreement shall mean a reference to the Loan Agreement as amended hereby.

         Section 6.03. Expenses of Bank. Borrower agrees to pay on demand all
reasonable costs and expenses incurred by Bank directly in connection with the
preparation, negotiation and execution of this Amendment and the other Loan
Documents executed pursuant hereto and any and all amendments, modifications,
and supplements thereto, including, without limitation, the costs and fees of
Bank's legal counsel, and all costs and expenses incurred by Bank in connection
with the enforcement or preservation of any rights under the Loan Agreement, as
amended hereby, or any other Loan Documents including, without limitation, the
reasonable costs and fees of Bank's legal counsel.

         Section 6.04. Severability. Any provision of this Amendment held by a
court of competent jurisdiction to be invalid or unenforceable shall not impair
or invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

         SECTION 6.05. APPLICABLE LAW. THIS AMENDMENT SHALL BE DEEMED TO HAVE
BEEN MADE AND TO BE PERFORMABLE IN AND SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF TEXAS.

         Section 6.06. Successors and Assigns. This Amendment is binding upon
and shall inure to the benefit of the parties hereto and their respective
successors, assigns, heirs, executors, and legal representatives, except that
none of the parties hereto other than Bank may assign or transfer any of its
rights or obligations hereunder without the prior written consent of Bank.

         Section 6.07. Counterparts. This Amendment may be executed in one or
more counterparts, each of which when so executed shall be deemed to be an
original, but all of which when taken together shall constitute one and the
same instrument.

         Section 6.08. Effect of Waiver. No consent or waiver, express or
implied, by Bank to or for any breach of or deviation from any covenant,
condition or duty by Borrower, shall be deemed a consent to or waiver of any
other breach of the same or any other covenant, condition or duty.




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         Section 6.09. Headings. The headings, captions, and arrangements used
in this Amendment are for convenience only and shall not affect the
interpretation of this Amendment.

         Section 6.10. Conflicting Provisions. If any provision of the Loan
Agreement as amended hereby conflicts with any provision of any other Loan
Document, the provision in the Loan Agreement shall control.

         SECTION 6.11. RELEASE. BORROWER AND EACH GUARANTOR HEREBY ACKNOWLEDGES
THAT NEITHER BORROWER NOR ANY GUARANTOR HAS ANY DEFENSE, COUNTERCLAIM, OFFSET,
CROSS-COMPLAINT, CLAIM OR DEMAND OF ANY KIND OR NATURE WHATSOEVER THAT CAN BE
ASSERTED TO REDUCE OR ELIMINATE ALL OR ANY PART OF BORROWER'S OR ANY
GUARANTOR'S LIABILITY TO REPAY THE INDEBTEDNESS OR TO SEEK AFFIRMATIVE RELIEF
OR DAMAGES OF ANY KIND OR NATURE FROM BANK. BORROWER AND EACH GUARANTOR HEREBY
VOLUNTARILY AND KNOWINGLY RELEASES AND FOREVER DISCHARGES BANK, ITS
PREDECESSORS, AGENTS, EMPLOYEES, SUCCESSORS AND ASSIGNS, FROM ALL POSSIBLE
CLAIMS, DEMANDS, ACTIONS, CAUSES OF ACTION, DAMAGES, COSTS, EXPENSES, AND
LIABILITIES WHATSOEVER, KNOWN OR UNKNOWN, ANTICIPATED OR UNANTICIPATED,
SUSPECTED OR UNSUSPECTED, FIXED, CONTINGENT, OR CONDITIONAL, AT LAW OR IN
EQUITY, ORIGINATING IN WHOLE OR IN PART ON OR BEFORE THE DATE THIS AGREEMENT IS
EXECUTED, WHICH BORROWER OR ANY GUARANTOR MAY NOW OR HEREAFTER HAVE AGAINST
BANK, ITS PREDECESSORS, AGENTS, EMPLOYEES, SUCCESSORS AND ASSIGNS, IF ANY, AND
IRRESPECTIVE OR WHETHER ANY SUCH CLAIMS ARISE OUT OF CONTRACT, TORT, VIOLATION
OF LAW OR REGULATIONS, OR OTHERWISE OR ARISE FROM ANY LOAN, INCLUDING, WITHOUT
LIMITATION, ANY CONTRACTING FOR, CHARGING, TAKING, RESERVING, COLLECTING OR
RECEIVING INTEREST IN EXCESS OF THE HIGHEST LAWFUL RATE, THE EXERCISE OF ANY
RIGHTS AND REMEDIES UNDER THE LOAN DOCUMENTS, AND/OR NEGOTIATION AND EXECUTION
OF THIS AMENDMENT.

         SECTION 6.12. ENTIRE AGREEMENT. THIS AMENDMENT, THE LOAN AGREEMENT AND
ALL OTHER LOAN DOCUMENTS EXECUTED AND DELIVERED IN CONNECTION WITH AND PURSUANT
TO THIS AMENDMENT AND THE LOAN AGREEMENT REPRESENT THE FINAL AGREEMENT BETWEEN
THE PARTIES AND MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS,
OR SUBSEQUENT ORAL AGREEMENTS OF THE PARTIES. THERE ARE NO UNWRITTEN ORAL
AGREEMENTS BETWEEN THE PARTIES.




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         EXECUTED as of the date first written above.

                                       Bank One, Texas, National Association


                                       By: /s/ BURTON FRENCH
                                          ------------------------------------
                                             Burton French, Vice President

                                       MedicalControl, Inc.


                                       By: /s/ ROBERT O. BROOKS
                                          -------------------------------------
                                             Robert O. Brooks, Executive Vice 
                                             President and Chief Operating 
                                             Officer

                           CONFIRMATION BY GUARANTORS

         Each of the undersigned, MedicalControl Network Solutions, Inc.,
Diversified Group Administrators, Inc. and ppoONE.com,inc. (f/k/a PPO
Management Solutions, Inc.), hereby (i) consents and agrees to the terms hereof
and agrees to be bound by this Amendment, (ii) consents, acknowledges, and
agrees to the execution, delivery, and performance by Borrower of this
Amendment, (iii) acknowledges and agrees that this Amendment does not affect,
diminish, waive, or release its obligations under the Unlimited Guaranty
executed by it on September 25, 1998, or under the Security Agreement executed
by it on September 25, 1998, (iv) ratifies and confirms its obligations
pursuant to such Unlimited Guaranty and such Security Agreement, and (v)
covenants, acknowledges, and agrees that it guaranties the repayment of the
Guaranteed Indebtedness, as provided in such Unlimited Guaranty.

                                       GUARANTORS:

                                       MedicalControl Network Solutions, Inc.


                                       By: /s/ ROBERT O. BROOKS
                                          ------------------------------------
                                             Robert O. Brooks, President and 
                                             Chief Executive Officer

                                       Diversified Group Administrators, Inc.


                                       By: /s/ DAVID C. BRAMER
                                          ------------------------------------
                                             David C. Bramer, President and
                                             Chief Executive Officer




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<PAGE>   11



                                       ppoONE.com, inc. f/k/a PPO Management
                                       Solutions, Inc.


                                       By: /s/ ROBERT O. BROOKS
                                          ------------------------------------
                                             Robert O. Brooks, President and
                                             Chief Executive Officer





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