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                                                                   EXHIBIT 10.21

                  FIRST AMENDMENT TO THE WATERS LEASE OF SPACE


         THIS FIRST AMENDMENT OF THE WATERS LEASE OF SPACE (this "First
Amendment") is made as of the 31 day of July, 2000, by and between THOMAS S.
SCHREIER,(the "Landlord"), and Hypertension Diagnostics, Inc. (the "Tenant").

                                   WITNESSETH

         A.       Landlord and Tenant entered into that certain Waters Lease of
                  Space dated as of October 24, 1997 (the "Lease"), pursuant to
                  which Tenant leased 6,947 square feet (the "Demised Premises")
                  in the building commonly known as The Waters Phase II, 2915
                  Waters Road, Eagan, Minnesota 55121 (the "Building").

         B.       Tenant desires to extend the term of the Lease and Landlord is
                  willing to accept such amendments upon terms and conditions
                  hereinafter set forth.

         NOW, THEREFORE, for good and valuable consideration, the receipt and
legal sufficiency of which are hereby acknowledged, it is hereby agreed as
follows:

         1.       TERM - The term of the Lease shall be extended for thirty-six
                  (36) months expiring October 31, 2003, unless sooner
                  terminated as set forth in the Lease or this First Amendment.

         2.       BASE RENT - Monthly Base Rent for the period November 1, 2000,
                  through October 31, 2003, shall be as follows:

<TABLE>
<CAPTION>
                  Period                                           Monthly Base Rent
                  ------------------------------------------------------------------
<S>                                                                <C>
                  November 1, 2000 through and                         $4,462.35
                  Including October 31, 2001

                  November 1, 2001 through and                         $4,607.10
                  Including October 31, 2002

                  November 1, 2002 through and                         $4,751.85
                  Including October 31, 2003
</TABLE>

3.                EARLY TERMINATION - Provided Tenant is not in default as to
                  any of the Terms and Conditions of the Lease, Tenant shall
                  have the right to terminate the Lease on either April 30, 2002
                  or January 31, 2003 with six (6) months prior written notice
                  to Landlord and payment at the time of notice of the following
                  early termination fee:

<TABLE>
<S>                                                                   <C>
                  If terminated on April 30, 2002                     $13,000.00
</TABLE>

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<TABLE>
<S>                                                                    <C>
                  If terminated on January 31, 2003                    $9,000.00
</TABLE>

         All other terms and conditions of the Lease shall remain in full force
and effect.

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


LANDLORD:                                   THOMAS S. SCHREIER


                                             /s/ Thomas S. Schreier
                                            ------------------------------------


TENANT:                                     HYPERTENSION DIAGNOSTICS, INC.
                                            (A Minnesota Corporation)

                                            By: /s/ James S. Murphy
                                                --------------------------------

                                            Its: Senior Vice President - Finance
                                                    and Chief Financial Officer