his review paper, including some unpublished results, summarizes some of the effects on sex differentiation in male fetuses of drugs which inhibit enzymes involved in steroid hormone synesis or of antiandrogens, especially cyproterone acetate. The former have only very slight effects on morphological sex differentiation. Cyproterone acetate is a very potent antimasculine agent in several species (rabbit, dog and probably others); in other species (rat, guinea pig, calf) the results obtained so far are at variance. Several apparently contradictory data pertaining to the stabilization and differentiation of the Wolffian ducts of the rat fetus are discussed.