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]dihydrotestosterone. The rat uterine homogenates showed higher activity than the minced tissue. The activity in human tissue was found to be less than rat tissue on a per unit weight basis. The human endometrium has been found to possess significantly higher activity than myometrium. These results support the hypothesis that androgens may exert their effect on uterus via 5 alpha-reduced metabolites.