Sch 13521 (4'–nitro–3'–trifluoromethylisobutyranilide) at daily doses from 1–50 mg/kg reduced seminal vesicles and ventral prostate wt of intact male rats treated orally for 1–3 weeks. No alterations in sex structures of female rats were observed with doses as high as 50 mg/kg. Sch 13521 antagonized the effects of testosterone, testosterone propionate, androstenedione, and dihydrotestosterone on seminal vesicles and ventral prostate wt in castrated rats as well as the effects of hCG in hypophysectomized rats. A comparative study in castrated rats revealed that Sch 13521 was equipotent to cyproterone acetate (CPA) as an antiandrogen. However, when Sch 13521 was given to gravid rats, days 16–19, it reduced the anogenital distance in male fetuses and altered the normal development of the scrotum and penis, but unlike CPA, Sch 13521 did not effect parturition time. In the majority of these male rats, hypospadias and vaginal tracts were present and ventral prostates and seminal vesicles were absent or markedly atrophied. Sch 13521 prevented the uterotrophic response elicited in ovariectomized rats by testosterone, androstenedione, or dihydrotestosterone but not that elicited by dehydroepiandrosterone. Administration of Sch 13521 for 74 days to mature rats suppressed ventral prostate and seminal vesicle wt in mature rats but did not interfere with libido or sexual potency. No androgenic, estrogenic, antiestrogenic, corticoid, progestational, antiprogestational, or antigonadotrophic activities were observed. (Endocrinology91: 427, 1972)