Rat pituitary 5α–reductase may use as substrate, in addition to testosterone, also progesterone, orticosterone or estrogenic hormones in in vitro experiments. 5α–reductase activity has been measured by dihydrotestosterone synthesis from testosterone–4-14C after incubation with adult male or female rat pituitaries. Castration has been followed by an increase in 5α–reductase activity. Subcutaneous injection of testosteronepropionate, but not of progesterone, estradiol, corticosterone, serotonin or dopamine hydrochloride, respectively, has been capable of maintaining pituitary 5α–reductase activity in gonadectomized males on a level similar to that in intact control animals. Ovariectomy has also been found to increase 5α–reductase activity in the pituitary, but the increase is slower than in males. As in males, of all hormones injected only testosterone has lowered 5α–reductase activity to control values. The results obtained indicate that testosterone is a regulator of 5α–reductase activity in the pituitary of male and female rats.(Endocrinology92: 1772, 1973)