Nuveen Select Tax-Free Income Portfolio fund's investment objective is current income exempt from regular federal income tax, consistent with preservation of capital. The Fund invests in municipal securities that are exempt from federal income taxes. The Fund invests at least 80% of its managed assets in municipal securities rated investment grade at the time of investment, or, if they are unrated, are judged by the manager to be of comparable quality. The Fund may invest up to 20% of its managed assets in municipal securities rated below investment quality or judged by the manager to be of comparable quality, of which up to 10% of its managed assets may be rated below B-/B3 or of comparable quality. The Fund may invest in inverse floating rate municipal securities, also known as tender option bonds. The Fund's use of tender option bonds to more efficiently implement its investment strategy may create up to 10% effective leverage.