The effect of age on in vivo tritiated testosterone (3H-T) metabolism and on epididymal 5 alpha-reductase and 3 alpha/beta-hydroxysteroid dehydrogenase activity has been studied on male Wistar rats. Following i.m. injection of 3H-T to 3 and 25 month old functionally hepatectomized rats, the accumulation of 5 alpha-androstan-17 beta-ol-3-one (DHT) by both the caput and cauda epididymidis, prostate, and seminal vesicles was significantly higher in the younger animals than in the old. The total conversion of 3H-T by both epididymal segments was unaltered with advancing age whereas a significant decrease was observed in both prostate and seminal vesicles. The ratio between 17 beta-hydroxy and 17-keto metabolites in the prostate and epididymis of both hepatectomized and intact animals decreased significantly with age. The ratio values for the seminal vesicles showed no significant age variation. The formation of total 5 alpha-reduced metabolites by epididymal homogenates showed a linear decrease with advancing age. The activity of 5 alpha-reductase fell from 16.1 to 10.3 ng/mg protein/30 min. 3 alpha/beta-hydroxysteroid dehydrogenase activity was unaffected by age. The reduced levels of active testosterone metabolites reported with old age are a result of a decrease in 5 alpha-reductase activity, the ultimate cause of which is most probably an age associated defect in the hypothalamic-pituitary-testicular feedback system.