Testosterone administration to orchiectomized rats inhibited hepatic Δ4–steroid hydrogenase activity and prolonged the in vivo half—life of corticosterone. In rats that were hypophysectomized as well as castrated, testosterone was without effect on corticosteroid metabolism. The actions of estradiol to stimulate A—ring reduction and hasten in vivo clearance of corticosterone in orchiectomized rats also required the presence of the pituitary gland. Testosterone administration to hamsters stimulated cortisol metabolism in vitro and in vivo but not in the absence of the pituitary. The effects of testosterone were fully manifested in adrenalectomized or thyroidectomized rats. The results indicate that the effects of the gonadal hormones on corticosteroid metabolism in rats and hamsters are manifested only in the presence of the pituitary gland. (Endocrinology92: 769, 1973)