The nonsteroidal anntiandrogen, flutamide (α-α-α-trifluoro-2-methyl-4′-nitro-m-propionotoluidide or SCH 13521), suppresses the retention of a specific 5α-dihydrotestosteronereceptor complex by cell nuclei of rat ventral prostate in vivo. At 1–10 µ-M concentrations, flutamide also inhibits the binding of 5α-dihydrotestosterone in vitro by a receptor protein in the prostate cytosol, and the nuclear retention of the androgen-receptor complex during the incubation of minced prostate. The nonantiandrogenic analogue, acetanilide, at 100 µM, does not show such an inhibitory effect. While flutamide and cyproterone are equipotent as antiandrogens in vivo, cyproterone is about 10 times more active than flutamide in inhibiting the receptor binding of 5 αdihydrotestosterone. This discrepancy may be due to a metabolic activation in the whole animal and' or inactivation of flutamide in the prostate. The suppressive action of flutamide (or its active metabolite) may be related to its geometrical structure, which is similar to that of 5α-dihydrotestosterone. (Endocrinology94: 1205, 1974)