OPTICAL SENSORS INC
LETTER AGREEMENT
<DOCUMENT>
<TYPE>EX-10.23
<SEQUENCE>8
<FILENAME>0008.txt
<DESCRIPTION>LETTER AGREEMENT
<TEXT>

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                                                                   EXHIBIT 10.23

March 10, 2001


Special Situations Fund III, L.P.
153 East 53rd Street, 55th Floor
New York, NY 10022


Gentlemen:

This letter is delivered to you with reference to (i) the Investment Agreement,
dated as of March 10, 2000 (the "Investment Agreement"), by and among Optical
Sensors Incorporated (the "Company"), Circle F Ventures LLC and Special
Situations Fund III, L.P. ("SSF"), (ii) the Convertible Promissory Note ("Note")
issued by the Company to SSF pursuant to the Investment Agreement and (iii) the
Warrants (individually, a "Warrant" and collectively, the "Warrants") issuable
by the Company to SSF upon conversion of the Note.

Section 3(a) of the Investment Agreement and the first sentence of Section 2.1
of the Note are hereby amended to the extent necessary to change the conversion
price of the Note to Twenty Five Thousand Dollars ($25,000) per Unit. Section
3(a) of the Investment Agreement is hereby amended to the extent necessary to
change the initial exercise price of the Warrant to $.50 per share.

Section 1 of the Investment Agreement and the first paragraph of the Note are
hereby amended to the extent necessary to extend the Maturity Date of the Note
to September 30, 2001.

Section 3(c) of the Investment Agreement is hereby amended and restated in its
entirety as follows:

     (c)  Conversion. The entire principal balance of the Note will convert into
          Units, without any action of the Investor, 30 days following written
          notice from the Company, of the earlier to occur of (i) the completion
          of a financing by the Company that results in gross proceeds to the
          Company of at least $2,500,000, (ii) the Company entering into a
          distribution or other strategic relationship with a third party under
          which the Company will receive value of at least $2,000,000 during the
          term of such relationship, or (iii) any combination of the
          transactions described in clauses (i) or (ii) which, in the aggregate,
          results in gross proceeds to the Company or under which the Company
          will receive at least $2,500,000.

The registration rights referred to in Section 7 of the Investment Agreement
shall cover all shares of Common Stock issuable upon conversion of the Notes and
exercise of the Warrants.

This letter shall be binding on the Company and SSF, but shall not amend the
Investment Agreement with respect to any other party thereto or amend the terms
of any notes or warrants
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Special Situations Fund III, L.P.
March 10, 2001
Page 2

issued under the Investment Agreement to any party other than SSF. In the event
of any inconsistency between the terms of this letter and the terms of the
Investment Agreement, the Note or any Warrant, the terms of this letter shall
govern.



OPTICAL SENSORS INCORPORATED           SPECIAL SITUATIONS FUND


/s/ Wesley G. Peterson                 /s/ David Greenhouse

Wesley G. Peterson                     David Greenhouse
Chief Financial Officer
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