<PAGE>

                                                                   Exhibit 10.26
                           LEASE AMENDMENT NUMBER FIVE


         THIS LEASE AMENDMENT NUMBER FIVE is made this second day of August,
1999, by and between the Port Authority of the City of St. Paul, a public body
corporate and politic, created pursuant to Chapter 469 of Minnesota Statutes,
herein called "Lessor," and Endocardial Solutions, Inc., a Delaware corporation,
herein called "Tenant."

                                    RECITALS

         WHEREAS, by a Lease dated September 15, 1993, Endocardial Therapeutics,
Inc. did lease from Lessor Suite 110E on the first floor in that certain office
building known as Energy Park Place East Building, located at 1350 Energy Lane,
St. Paul, Minnesota; and by the Lease Modification and Extension Agreement dated
February 6, 1995, Tenant did lease Suite 107E and extended term; and by the
Lease Amendment Number Two dated May 16, 1995, the tax clause was modified; and
by Lease Amendment Number Three dated June 4, 1996, Tenant did Lease Suite 113E
and extended term; and by a notice letter dated March 31, 1997 and acceptance
letter dated June 16, 1997, Tenant exercised its option to extend, and by Lease
Amendment Number Four dated January 23, 1998, Tenant did lease Suite 207. Said
space consists of approximately 21,535 rentable square feet, is shown in Exhibit
A attached to said Lease, and is herein called the "Premises" and the "Existing
Space."

         WHEREAS, the term of said Lease expired on March 31, 1999 and continued
month-to-month thereafter; and

         WHEREAS, the parties wish to extend term, add certain space to the
Premises, and make certain changes to said Lease.

                                    AGREEMENT

         NOW, THEREFORE, in consideration of the mutual covenants contained
herein and in said Lease, the parties hereto agree as follows:

         1.   ENLARGEMENT OF PREMISES: Effective as of October 1, 1999, the
description of the Premises contained in said lease is amended so as to add
Suite 106E, consisting of approximately 2,138 rentable square feet, and
effective as of October 1, 1999, Suite 105E is added, consisting of 4,893
rentable square feet, and effective October 1, 1999, Suite 104E is added,
consisting of 1,834 rentable square feet, and effective September 30, 1999 Suite
207E is subtracted consisting of 4,056 rentable square feet, and hereinafter
called the "Additional Space." As of October 1, 1999, the total space
(hereinafter called the "Enlarged Premises") leased to Tenant under said Lease
shall consist of a total area of approximately 26,344 rentable square feet. The
Enlarged Premises are generally shown on the floor plan attached hereto as
Exhibit "A," which exhibit is made a part hereof by this reference. As of
October 1, 1999, that certain Exhibit "A" attached to said Lease is hereby
superseded (by Exhibit "A" attached hereto) and of no further force and effect.


                                      -1-

<PAGE>

         2.   NET BASE RENT: Effective as of the dates below, the net base rent
payable shall be as follows for the extended term:

<TABLE>
<CAPTION>

---------------------------------------------------------------------------------
                                                                     NET BASE
        DATES                    SUITE                  SF         RENT/RSF/YR
---------------------------------------------------------------------------------
   <S>                       <C>                     <C>          <C>
    4/1/99-9/30/99           110E/107E/113E           17,479          $9.00
                                  207E                 4,056          $9.00
                                                      ------
                                 Total                21,535
---------------------------------------------------------------------------------
   10/1/99-3/31/00           110E/107E/113E           17,479          $9.00
                             104E/105E/106E            8,865          $9.00
                                                      ------
                                 Total                26,344
---------------------------------------------------------------------------------
    4/1/00-3/31/01           110E/107E/113E           17,479          $9.50
                             104E/105E/106E            8,865          $9.50
                                                      ------
                                 Total                26,344
---------------------------------------------------------------------------------
</TABLE>

The base rent, as increased herein, shall continue to be payable in advance by
the first of each month and subject to operating expenses and taxes as well as
its separately metered utilities. Tenant is solely responsible for its own
janitorial and garbage removal and disposal for the Enlarged Premises. If the
Additional Space is added to the Premises on a date prior to October 1, 1999,
the resulting increase in rent shall be prorated for such partial month.

              3.    EXTENSION  OF LEASE  TERM:  The term of the Lease is
hereby extended for a period of two (2) years, from April 1, 1999 to and
including March 31, 2001.

              4.    CONDITION OF EXISTING AND ADDITIONAL SPACE/TENANT'S WORK:
Tenant is presently in possession of the Existing Premises and accepts same in
its present condition as of the date hereof. Tenant has inspected the Additional
Space including but not limited to HVAC capacity and distribution, electrical
capacity and distribution, and telecommunication capacity and distribution and
accepts same as of the date hereof. Any improvements to be done by Tenant to the
Additional Space shall be at Tenant's sole cost and expense, and shall be
performed in accordance with the provisions of said Lease. Tenant agrees that
any additional HVAC modifications must comply with Energy Center regulations.
Any supplemental, non Energy Center connected, existing HVAC units must remain
as designed prior to Tenant's occupancy. Pursuant to Sections 5 and 6 of this
Lease and provided Lessor approves plan, and contractors are licensed, bonded
and insured, Tenant shall proceed to complete certain improvements to Suites
106E, 105E and 104E as shown on attached Exhibit E. Provided Tenant is not in
default, and in consideration of Tenant accepting the Premises in "as-is"
condition and completing its own tenant improvements, Lessor agrees to reimburse
Tenant for the cost of building standard improvements, including architectural
fees, permits, performance and payment bonds, demolition, and construction
financing, prior to November 1, 1999. The tenant improvement reimbursement shall
be given to Tenant in the form of Net Base Rent Abatement. Upon completion of
tenant improvements, Tenant shall present Lessor with copies of all invoices,
contractor unconditional lien releases and a complete set of "As-Built"
drawings. Then, Tenant and Lessor agree to execute an amendment to this Lease
setting forth the amount of Rent Abatement which shall be equivalent to the
actual amount Tenant spends on building standard improvements for Suites 106E,
105E and 104E, not to exceed $57,497.00 in Net Base Rent. If Tenant does not
improve Suites 106E, 105E and 104E by said amount by November 1, 1999,


                                      -2-

<PAGE>

said rent credit is null and void. Any improvement costs amount over this
allowance is Tenant's sole responsibility.

          5.  FINANCIAL STATEMENT: Attached as Exhibit "F" are Tenant's updated
audited corporate financial statements.

          6.  MISCELLANEOUS:

              (a)   The provisions of this Lease Amendment shall be fully
applicable to the Enlarged Premises and shall remain in full force and effect
for the duration of the term of said Lease, as extended herein.

              (b)   Except as otherwise set forth herein, all of the terms and
conditions of said Lease shall remain in full force and effect, and shall be
fully applicable to the Additional Space as well as the Existing Space,
throughout the duration of the term of said Lease, as extended herein. Said
Lease, as amended herein, constitutes the entire agreement between the parties
hereto, and no further modification of said Lease shall be binding unless
evidenced by an agreement in writing signed by Lessor and Tenant.

              (c)   The captions and paragraph numbers appearing in the
Amendment are inserted only as a matter of convenience and in no way define,
limit, construe, affect or describe the scope or intent of the provisions in
this Amendment.

         7.   This Lease Amendment Number Five will not be in effect until duly
signed by Lessor and Tenant.

IN WITNESS WHEREOF, Lessor and Tenant have executed this Lease Amendment Number
Five as of the day and year first above written.

LESSOR:                                       TENANT:

PORT AUTHORITY OF THE CITY OF SAINT PAUL      ENDOCARDIAL SOLUTIONS, INC.

By:  /s/ Kenneth R. Johnson                   By:  /s/ Leota Pearson
    ---------------------------------            -----------------------------
Its: President                                Its: VP Finance & CFO
    ---------------------------------            -----------------------------


By:                                           By:
    ---------------------------------            -----------------------------
Its:                                          Its:
    ---------------------------------            -----------------------------


--------------------------------------------------------------------------------
CONSULT YOUR ATTORNEY: This document has been prepared for approval by your
attorney. No representation or recommendation is made by Broker as to the legal
sufficiency, legal effect, or tax consequence of this document or the
transaction to which it relates. These are questions for your attorney and
financial advisors.
--------------------------------------------------------------------------------

                                      -3-