The metabolism of 19-norandrostenedione and [3H] 19-nortestosterone was examined in porcine Leydig cell preparations in the absence, or presence, of a 4-azasteroid inhibitor of 5 alpha-reductase. Evidence for a major production of 5 alpha-estrane-3 beta, 17 beta-diol and 3 beta-hydroxy-5 alpha-estran-17-one, as sulfo-conjugated steroids, was obtained by HPLC and gas-chromatography/mass spectrometry. The 4-aza-steroid clearly reduced the formation of both of the above 5 alpha-reduced products from the 19-norandrogens. From the HPLC profiles of the radioactive metabolites, it was also concluded that estrogen secretion was increased significantly by exposure of the cells to the 5 alpha-reductase inhibitor.