The action of the endogenous steroid epitestosterone administered to castrated male mice substituted with testosterone propionate is manifested by reduced weight increments and a reduced relative weight of their seminal vesicles and kidneys. Epitestosterone in vitro displaces androgens from their bond with receptors in cytosol from rat prostates and markedly inhibits the testosterone transformation to the more effective androgen dihydrotestosterone. Epitestosterone can be thus defined as a true endogenous antiandrogen; to its action at the receptor level a potent inhibitory effect on 5 alpha-reductase must be added.