Oxidation and reduction of 4-androstene-3,17-dione (androstenedione), 17 beta-hydroxy-4-androsten-3-one (testosterone), 17 beta-hydroxy-5 alpha-androstan-3-one (DHT), 5 alpha-androstan-3 alpha,17 beta-diol (3 alpha-A'diol) and 5 alpha-androstane-3 beta,17 beta-diol (3 beta-A'diol) were measured in homogenates from ventral (VP), dorsal (DP) and lateral prostate (LP), the coagulating gland (CG) and seminal vesicle (SV) of the intact sexually mature rat using NAD(H) or NADP(H) as cofactors. The specific activity of the various enzymes varied significantly between the different organs. 5 alpha-Reductase activity was highest in the DP and the CG, and undetectable in the LP. 17 beta-Hydroxysteroid oxidoreductase (17 beta-HSOR) activity was mainly confined to the LP. 3 alpha-Hydroxysteroid oxidoreductase (3 alpha-HSOR) activity was also highest in the LP. In the VP the highest 3 alpha-HSOR activity was recorded using NAD(H) as cofactor. In the other organs, similar or higher enzymatic activities were measured using NADP(H) as added cofactor. 3 beta-Hydroxysteroid oxidoreductase (3 beta-HSOR) activity was high in the LP and low or undetectable in the other tissues. Our results indicate that isoenzymes of 3 alpha-HSOR, 3 beta-HSOR and 17 beta-HSOR are present in the accessory sex organs of the rat.