Polar metabolites extracted from the effluents of viable porcine uterine strips superfused with either 6,7-3H-estrone or 6,7-3H-estradiol were identified as a 1:1 mixture of 6 alpha-hydroxyestrone and 7 alpha-hydroxyestrone by paper chromatography in various systems, derivatization and crystallizations to a constant specific activity. The hydroxylated compounds are the only derivatives detected after estrone superfusion. The major metabolite of estradiol released in short-time experiments is estrone followed by its 6 alpha- and 7 alpha-hydroxylated derivatives.