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                                                                EXHIBIT 10.21.1




                             SECURED PROMISSORY NOTE


$__________                                                    November 2, 2000


         FOR VALUE RECEIVED, this Secured Promissory Note (this "Note") is made
by Probex Corp., a Delaware corporation ("Maker"), to _________________________
_________________________ ("Payee"). This Note is one of the "Notes" as defined
in, and is entitled to the benefits of, that certain Security Agreement (the
"Security Agreement"), dated as of the date hereof, by and among Maker, Payee,
_____________________________________________________, _______________________
_____________________________________________.

         1. Payments. Maker hereby promises to pay to the order of Payee the
principal sum of ______________________________________ Dollars ($_________) at
_______________, ___________________________, or such other place as the holder
hereof may designate from time to time in writing, in lawful money of the United
States of America and in immediately available funds, together with interest on
the unpaid principal balance hereof at the rate provided herein from the date of
this Note until payment in full of the indebtedness evidenced by this Note. This
Note and all accrued and unpaid interest shall be due and payable in one lump
sum on January 15, 2001. Any payment made under this Note shall be applied first
to interest accrued and unpaid on the outstanding principal balance as of such
date of payment and then to the outstanding principal balance due hereunder. If
any required payment falls due on a Saturday, Sunday or a national or state bank
holiday in Texas, then such date shall be extended to the next succeeding day
that is not a Saturday, Sunday or national or state bank holiday.

         2. Interest Rate. The principal amount outstanding from time to time
hereunder shall bear interest calculated on the basis of a 365-day year, at a
rate equal to twelve percent (12%) per annum.

         3. Voluntary Prepayment. This Note may be prepaid, in whole or in part,
without premium or penalty. All prepayments shall be applied first to accrued
interest and then to principal.

         4. Mandatory Prepayment. Maker shall be required to prepay this Note
upon the cumulative receipt by Maker or its subsidiaries of immediately
available funds equal to or greater than $5,000,000 derived from any type of
equity and/or debt financing benefiting Maker. As long as this Note remains
outstanding, Maker shall accept any reasonable financing proposal when presented
to Maker; provided, however, such proposal's reasonableness is to be determined
by the majority of the voting members of the board of directors of Maker acting
in the exercise of their business judgment.

         5. Event of Default. If any of the following events ("Events of
Default") shall occur:

            (a) Maker shall fail to pay any principal of, or any interest on,
this Note or any other amount payable under this Note, when and as the same
shall become due and payable;


            (b) any representation or warranty made or deemed made by or on
behalf of Maker in the Security Agreement, or any amendment or modification
thereof or waiver thereunder, or in any report, certificate, financial statement
or other document furnished pursuant to or in connection the Security Agreement,
or any amendment or modification thereof or waiver thereunder, shall prove to
have been incorrect when made or deemed made;


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            (c) Maker shall fail to observe or perform any covenant, condition
or agreement contained in the Security Agreement;

            (d) Maker shall fail to make any payment (whether of principal or
interest and regardless of amount) in respect of any indebtedness individually
or in the aggregate in excess of $100,000 ("Material Indebtedness"), when and as
the same shall become due and payable;

            (e) any event or condition occurs that results in any Material
Indebtedness of Maker becoming due prior to its scheduled maturity or that
enables or permits (with or without the giving of notice, the lapse of time or
both) the holder or holders of any Material Indebtedness of Maker or any trustee
or agent on its or their behalf to cause any Material Indebtedness of Maker to
become due, or to require the prepayment, repurchase, redemption or defeasance
thereof, prior to its scheduled maturity;

            (f) an involuntary proceeding shall be commenced or an involuntary
petition shall be filed seeking (i) liquidation, reorganization or other relief
in respect of Maker or its debts, or of a substantial part of its assets, under
any federal, state or foreign bankruptcy, insolvency, receivership or similar
law now or hereafter in effect or (ii) the appointment of a receiver, trustee,
custodian, sequestrator, conservator or similar official for Maker or for a
substantial part of its assets, and, in any such case, such proceeding or
petition shall continue undismissed for 60 days or an order or decree approving
or ordering any of the foregoing shall be entered;

            (g) Maker shall (i) voluntarily commence any proceeding or file any
petition seeking liquidation, reorganization or other relief under any federal,
state or foreign bankruptcy, insolvency, receivership or similar law now or
hereafter in effect, (ii) consent to the institution of, or fail to contest in a
timely and appropriate manner, any proceeding or petition described in clause
(f) of this Section, (iii) apply for or consent to the appointment of a
receiver, trustee, custodian, sequestrator, conservator or similar official for
Maker or for a substantial part of its assets, (iv) file an answer admitting the
material allegations of a petition filed against it in any such proceeding, (v)
make a general assignment for the benefit of creditors or (vi) take any action
for the purpose of effecting any of the foregoing;

            (h) Maker shall become unable, admit in writing its inability or
fail generally to pay its debts as they become due;

            (i) one or more judgments for the payment of money in an aggregate
amount in excess of $100,000 shall be rendered against Maker and the same shall
remain undischarged for a period of thirty (30) days during which execution
shall not be effectively stayed, or any action shall be legally taken by a
judgment creditor to attach or levy upon any assets of Maker to enforce any such
judgment;

            (j) any lien purported to be created under the Security Agreement
shall cease to be, or shall be asserted by Maker or any affiliate thereof not to
be, a valid and perfected lien on the Collateral (as defined in the Security
Agreement), with the priority required by the Security Agreement, except (i) as
a result of the sale or other disposition of the Collateral in a transaction
permitted under the Security Agreement or (ii) as a result of Payee's failure to
maintain possession of any promissory notes or other instruments delivered to it
under the applicable Security Agreement;

            (k) there shall occur, in the judgment of Payee, a material adverse
change in the business, assets or prospects of Maker after the date hereof;

            (l) there shall occur any material loss, theft, damage or
destruction of any of Maker's property or assets not fully covered by insurance;
or

            (m) there shall occur a cessation of a substantial part of the
business of Maker for a period which significantly effects its respective
capacity to continue its business on a profitable basis; or Maker shall suffer
the loss or revocation of any license or permit now held or hereafter acquired
by it which is necessary to the continued or lawful operation of its respective
business; or Maker shall be enjoined, restrained or in any way prevented by
court, governmental or administration order from


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conducting all or any material part of its respective business affairs; or any
material part of Maker's property shall be taken through condemnation or the
value of such property shall be materially impaired through condemnation;

then, and in every such event (other than an event described in clause (f) or
(g) of this Section), and at any time thereafter during the continuance of such
event, the Payee may declare the then outstanding amounts hereunder to be due
and payable in whole (or in part, in which case any principal not so declared to
be due and payable may thereafter be declared to be due and payable), and
thereupon the principal of the amounts hereunder so declared to be due and
payable, together with accrued interest thereon and all other obligations of
Maker accrued hereunder, shall become due and payable immediately, without
presentment, demand, protest or other notice of any kind, all of which are
hereby waived by Maker; and in case of any event with respect to Maker described
in clause (f) or (g) of this Section, the principal amounts hereunder then
outstanding, together with accrued interest thereon and all other obligations of
Maker accrued hereunder, shall automatically become due and payable, without
presentment, demand, protest or other notice of any kind, all of which are
hereby waived by Maker.

         6. Remedies Cumulative. The remedies of Payee, as provided herein,
shall be cumulative and concurrent, and may be pursued singularly, successively
or together, at the sole discretion of Payee, and may be exercised as often as
occasion therefor shall arise. No act of omission or commission of Payee,
including specifically any failure to exercise any right, remedy or recourse,
shall be deemed to be a waiver or release of the same, such waiver or release to
be effected only through a written document executed by Payee and then only to
the extent specifically recited therein. A waiver or release with reference to
any one event shall not be construed as continuing, as a bar to, or as a waiver
or release of, any subsequent right, remedy or recourse as to a subsequent
event.

         7. Notices. Except as otherwise provided herein, any notice or demand
which, by the provisions hereof, is required or which may be given to or served
upon Maker or Payee shall be in writing and, if by telecopy, shall be deemed to
have been validly served, given or delivered when transmitted with a copy
immediately mailed by registered or certified mail, if by personal delivery,
shall be deemed to have been validly served, given or delivered upon actual
delivery and, if mailed, shall be deemed to have been validly served, given or
delivered three (3) business days after deposit in the United States mails, as
registered or certified mail, with proper postage prepaid and addressed to the
party to be notified, at the addresses last given in writing by Maker and Payee.

         8. Successors and Assigns. This Note shall be binding upon Maker and
its successors and assigns (including, without limitation, a receiver, trustee
or debtor-in-possession of or for Maker) and shall inure to the benefit of Payee
and its successors and assigns. Maker may not assign its rights hereunder
without the prior written consent of Payee, in its sole discretion, other than
by operation of law. Payee may assign all or a part of its interest in this Note
or its rights hereunder to any party without the prior written consent of Maker,
in its sole discretion, other than by operation of law.

         9. GOVERNING LAW. THIS NOTE SHALL BE DEEMED A CONTRACT AND INSTRUMENT
MADE UNDER THE LAWS OF THE STATE OF TEXAS AND ACCEPTED BY PAYEE IN SAID STATE,
AND ANY AND ALL CLAIMS, DEMANDS OR ACTIONS IN ANY WAY RELATING THERETO OR
INVOLVING ANY DISPUTE BETWEEN ANY OF THE PARTIES TO THIS NOTE, WHETHER ARISING
IN CONTRACT OR TORT, AT LAW, IN EQUITY OR STATUTORILY, SHALL BE CONSTRUED AND
ENFORCED IN ACCORDANCE WITH AND/OR GOVERNED BY THE LAWS OF THE STATE OF TEXAS
(EXCEPTING ITS CHOICE OF LAW RULES) AND THE LAWS OF THE UNITED STATES OF
AMERICA.


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         10. Severability. If any provisions of this Note or any payments
pursuant to the terms hereof shall be invalid or unenforceable to any extent,
the remainder of this Note and any other payments hereunder shall not be
affected thereby and shall be enforceable to the greatest extent permitted by
law. Furthermore, in lieu of such invalid or unenforceable provisions, there
shall be added automatically as part of this Note, a provision or provisions as
similar in its or their terms to such invalid or unenforceable provisions as may
be possible and be legal, valid and enforceable.

         11. No Oral Agreements. This Note and the Security Agreement as written
represent the final agreement between Maker and Payee with respect to the
matters contained therein and may not be contradicted by evidence of prior,
contemporaneous or subsequent oral agreements between Maker and Payee. There are
no unwritten agreements between Maker and Payee.


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         IN WITNESS WHEREOF, Maker has executed and delivered this Note as of
the date and year first above written.

                                            MAKER:


                                            PROBEX CORP.,
                                            a Delaware corporation

                                            By:
                                                -------------------------------
                                            Name:
                                                 ------------------------------
                                            Title:
                                                   ----------------------------


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