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                                                                   EXHIBIT 10.18


THIS INSTRUMENT HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED, AND ANY STATE SECURITIES LAWS IN RELIANCE UPON APPLICABLE EXEMPTIONS
THEREUNDER. THIS INSTRUMENT MAY CONSTITUTE A "SECURITY" FOR THE PURPOSES OF SUCH
LAWS, AND, AS SUCH, MAY NOT BE FURTHER SOLD OR TRANSFERRED BY THE HOLDER IN THE
ABSENCE OF AN EFFECTIVE REGISTRATION UNDER SUCH LAWS OR AN EXEMPTION THEREUNDER
APPLICABLE TO SUCH SALE OR TRANSFER.


                                   CONVERTIBLE
                                 PROMISSORY NOTE


$1,500,000.00                     Dallas, Texas                  August 30, 2000


         FOR VALUE RECEIVED, the undersigned, PROBEX CORP., a Colorado
corporation ("Maker"), promises to pay to the order of General Conference
Corporation of Seventh-day Adventists ("Payee"), at 12501 Old Columbia Pike,
Silver Spring, MD 20904, or such other place as Payee may designate in writing,
in lawful money of the United States of America, the principal sum of One
Million Five Hundred Thousand AND NO/100 DOLLARS ($1,500,000).

         The outstanding principal balance of this Note shall bear interest at a
rate of ten percent (10%) per annum accruing from the date hereof on the unpaid
indebtedness until paid in full. Interest shall be calculated on the basis of a
360 day year and actual days elapsed.

         The principal balance of this Note shall be due and payable in one
installment of One Million Five Hundred Thousand AND NO/100 DOLLARS
($1,500,000), plus and all accrued and unpaid interest thereon, on October 31,
2000 (the "Maturity Date").

         Maker shall have the right to prepay, at any time without premium or
penalty, the entire outstanding principal balance of this Note and the accrued
interest thereon. Any such prepayment shall first be applied to accrued and
unpaid interest, and the balance of any such prepayment shall be applied to the
outstanding principal balance of this Note.

         This Note is unsecured and is a general obligation of Maker.

         Upon the completion of any offering by Maker of shares of its common
stock aggregating gross proceeds of at least Thirteen Million Five Hundred
Thousand Dollars ($13,500,000) (a "Qualified Offering"), Maker shall have the
right to require Payee to convert the aggregate unpaid principal balance and
unpaid interest into shares of common stock of Maker at the same valuation and
upon the same terms that are received by Maker in the Qualified Offering. To
effect such conversion, Maker shall provide notice to Payee, which notice shall
include the terms of the Qualified Offering. Upon receipt of such notice, Payee
shall tender this Note to Maker at its offices in Dallas, Texas. Upon the giving
of the notice of conversion as provided above, no further


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interest shall accrue upon the converted indebtedness hereof, and upon receipt
of this Note, Maker shall issue to Payee a certificate evidencing the shares of
Maker's common stock to which Payee is entitled in proper form, and shall
release Payee from all obligations under that certain Suitability Letter and
Subscription Agreement dated June 16, 2000, pursuant to which Payee had
subscribed for the purchase of One Million Five Hundred Thousand Dollars
($1,500,000) of common stock of Maker.

         In the event that Maker has not completed a Qualified Offering, Maker
shall have the right to require Payee to convert the aggregate unpaid principal
balance and unpaid interest into shares of common stock of Maker at a valuation
per share of common stock equal to the lesser of (i) ninety percent (90%) of the
average of the closing prices of the common stock, as reported on the American
Stock Exchange, for the ten (10) trading days preceding the date Maker provides
Payee with a notice of conversion, and (ii) $2.00. In the event Maker shall fail
to pay the outstanding principal balance and accrued interest thereon in
accordance with the terms of this Note, after notice and the expiration of any
applicable cure periods, this Note, including all unpaid principal and all
accrued and unpaid interest, shall automatically be converted into shares of
common stock at the valuation per share set forth in this paragraph (except the
date for calculation of the average closing price of the common stock shall be
the date of expiration of any applicable cure periods). Any conversion pursuant
to this paragraph shall otherwise be effected in the same manner as set forth in
the preceding paragraph.

         In the event of any stock dividend, split, combination or
reclassification directly affecting the then outstanding common stock of Maker,
the then effective conversion rate at which the indebtedness of this Note may be
converted into shares of common stock shall be proportionately adjusted, upward
or downward, to prevent dilution or enlargement of the rights of the holder
hereof, effective at the close of business on the date of such dividend, split,
combination or reclassification.

         Payee acknowledges that this Note is non-negotiable and shall not be
transferred or assigned by Payee. The shares of Maker 's common stock to be
issued upon conversion of this Note shall not be registered pursuant to the
Securities Act of 1933 or any state securities law or regulation. The
certificates evidencing such shares shall bear an appropriate legend to the
effect that such shares have not been registered under the Securities Act of
1933 nor under the securities laws of any state and that such shares may not be
sold within the United States of America unless so registered or unless Maker
shall receive an opinion of counsel satisfactory to it that such registration is
unnecessary.

         No fractional shares of common stock shall be issued upon conversion of
this Note. Instead of any fractional share which would otherwise be issuable
upon conversion, Maker will pay a cash adjustment with respect to such
fractional share in an amount equal to the same fraction of the then effective
conversion rate.

         The Board of Directors of Maker, or a committee established by it,
shall have the right from time to time to adopt specific rules of procedure to
carry out the full intent of the conversion


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provisions of this Note and to do all reasonable acts therefor; provided that
such rules and acts shall not violate the specific terms of this Note.

         Maker shall at all times reserve and hold available sufficient shares
of common stock to satisfy all conversion rights of this Note and other debt
instruments of the same class. Shares deliverable upon the conversion of the
indebtedness of this Note shall, at delivery, be fully paid and nonassessable,
free from all taxes, liens and charges arising out of their issuance.

         This Note does not entitle Payee to any voting rights or other rights
as a shareholder of Maker, or to any other rights whatsoever except the rights
herein expressed. No dividends with respect to the common stock are payable or
will accrue on this Note or the shares of common stock into which this Note may
be converted until, and except to the extent that, the conversion right granted
in this Note is exercised.

         As used herein, the term "Event of Default" shall mean the occurrence
and continuation of any of the following events:

                  (a) The failure of Maker to make any payment with respect to
         this Note when due, and such failure continues for a period of then
         (10) days after receipt of written notice of such nonpayment; or

                  (b) The adjudication of Maker as bankrupt, or the taking of
         any voluntary action by Maker seeking relief under any provision of the
         federal Bankruptcy Code or under any other debtor relief law.

         Upon the occurrence and during the continuation of an "Event of
Default," unless this Note shall have been automatically converted as provided
for herein, Payee may, at its option, without further notice of nonpayment,
demand for payment, presentment for payment, notice of intention to accelerate
maturity, notice of acceleration of maturity, or any other notice or demand of
any kind to Maker, declare the entire unpaid principal balance and accrued
interest on this Note to be immediately due and payable, at which time such
amounts shall become immediately due and payable. Payee may exercise this option
to accelerate maturity hereof during any default by Maker regardless of any
prior forbearance by Payee.

         If Payee expends any effort in any attempt to enforce payment of all or
any part or installment of any sum due Payee hereunder, or if this Note is
placed in the hands of an attorney for collection, or if it is collected through
any legal proceedings, Maker agrees to pay all collection costs and fees
incurred by Payee, including reasonable attorneys' fees.

         All agreements between Maker and Payee, whether now existing or
hereafter arising and whether written or oral, are expressly limited so that in
no contingency or event whatsoever, whether by reason of acceleration of the
maturity of this Note or otherwise, shall the amount paid or agreed to be paid
to the holder of this Note for the use, forbearance, or detention of the funds
advanced pursuant to this Note or for the performance or payment of any covenant
or obligation contained herein or in any other document evidencing, securing or
pertaining to this Note, exceed


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the maximum amount permitted under applicable law. If from any circumstance
whatsoever fulfillment of any provision hereof or of any such other document, at
the time performance of such provision shall be due, shall involve transcending
the limit of validity prescribed by applicable law, then ipso facto, the
obligation to be fulfilled shall be reduced to the limit of such validity, and
if from any circumstance the holder hereof shall ever receive anything of value
deemed excess interest by applicable law, an amount equal to any such excess
interest shall be applied to the reduction of the principal amount owing under
this Note, and not to the payment of interest, or if such excess interest
exceeds the unpaid principal balance of this Note, such excess interest shall be
refunded to Maker. All sums paid or agreed to be paid to any holder of this Note
for the use, forbearance or detention of any funds advanced pursuant to this
Note shall, to the extent permitted by applicable law, be amortized, prorated,
allocated and spread throughout the full term of this Note until payment in full
so that the rate of interest on account of the indebtedness evidenced by this
Note is uniform throughout the term hereof. The terms and provisions of this
paragraph shall control and supersede every other provision of all agreements
between Maker and any holder of this Note.

         This Note shall be governed and construed in accordance with the laws
of the State of Texas and the applicable laws of the United States of America.

         Maker and each surety, guarantor, endorser, and other party ever liable
for payment of any sums of money payable on this Note jointly and severally
waive notice, presentment, demand for payment, protest, notice of protest and
nonpayment or dishonor, notice of acceleration, notice of intent to accelerate,
notice of intent to demand, diligence in collecting, grace, and all other
formalities of any kind, and consent to all extensions without notice for any
period or periods of time and partial payments, before or after maturity, and
any impairment of any collateral securing this Note, all without prejudice to
the holder. The holder shall similarly have the right to deal in any way, at any
time, with one or more of the foregoing parties without notice to any other
party, and to grant any such party any extensions of time for payment of any of
said indebtedness, or to release or substitute part or all of the collateral
securing this Note, or to grant any other indulgences or forbearance whatsoever,
without notice to any other party and without in any way affecting the personal
liability of any party hereunder.

         THIS NOTE AND ALL OTHER INSTRUMENTS, DOCUMENTS AND AGREEMENTS EXECUTED
AND DELIVERED BY MAKER IN CONNECTION WITH THE INDEBTEDNESS EVIDENCED BY THIS
NOTE EMBODY THE FINAL, ENTIRE AGREEMENT OF MAKER AND PAYEE WITH RESPECT TO THE
INDEBTEDNESS EVIDENCED BY THIS NOTE AND SUPERSEDE ANY AND ALL PRIOR COMMITMENTS,
AGREEMENTS, REPRESENTATIONS AND UNDERSTANDINGS, WHETHER WRITTEN OR ORAL,
RELATING TO THE INDEBTEDNESS EVIDENCED BY THIS NOTE AND MAY NOT BE CONTRADICTED
OR VARIED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL AGREEMENTS OR
DISCUSSIONS OF MAKER AND PAYEE. THERE ARE NO ORAL AGREEMENTS BETWEEN MAKER AND
PAYEE.


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                          [SIGNATURE ON FOLLOWING PAGE]




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                                       PROBEX CORP.,
                                       a Colorado corporation


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