Certification Pursuant to 18 U.S.C. 1350, as adopted pursuant to Section 906 of
the Sarbanes-Oxley Act of 2002; provided by the Chief Executive Officer and
Chief Financial Officer, based on each such officer's knowledge and belief.

The undersigned officers of Nuveen Select Tax-Free Income Portfolio (the
"Fund"), certify that, to the best of each such officer's knowledge and belief:

     1.   The Form N-CSR of the Fund for the period ended March 31, 2008 (the
          "Report") fully complies with the requirements of Section 13(a) or
          15(d) of the Securities Exchange Act of 1934; and

     2.   The information contained in the Report fairly presents, in all
          material respects, the financial condition and results of operations
          of the Fund.

Date: June 6, 2008
      -----------------------

                                           /s/ Gifford R. Zimmerman
                                               ---------------------------------
                                               Gifford R. Zimmerman
                                               Chief Administrative Officer
                                               (principal executive officer)

                                           /s/ Stephen D. Foy
                                               ---------------------------------
                                               Stephen D. Foy
                                               Vice President, Controller
                                               (principal financial officer)