Incubations of adult rat uterine tissue and human endometrial and myometrial homogenates with [1,2,6,7-3H]testosterone resulted in the formation of 5.alpha.-[3H]dihydrostestosterone. The rat uterine homogenates showed higher activity than the minced tissue. The activity of human tissue was less than rat tissue on a per unit weight basis. The human endometrium possessed significantly higher activity than the myometrium. These results support the hypothesis that androgens may every their effect on the uterus via 5.alpha.-reduced metabolities.