The hormonal effects after a 10‐day administration of a 4‐azasteroid inhibitor of 5α‐reductase, MK‐0963 (previously L‐654,066), were evaluated in 35 healthy male volunteers in an increasing‐dose, five‐panel design. Marked suppression of serum dihydrotestosterone was observed after the once‐daily administration at each active dose level (placebo, 0.1, 0.5, 1.0, 10, and 25 mg). Maximum dihydrotestosterone suppression occurred at doses greater than or equal to 10 mg. The mean percentage (±SE) decreases in dihydrotestosterone at 24 hours after the last dose in the groups treated with the 10 and 25 mg doses were 78% ± 4.9% and 80% ± 2.9%, respectively. The 25 mg dose maintained a dihydrotestosterone suppression of at least 70% for more than 6 days after the last dose. No consistent changes in serum testosterone were noted. This study shows that administration of multiple doses of MK‐0963 results in a substantial suppression of serum dihydrotestosterone with no consistent influence on serum testosterone concentrations.Clinical Pharmacology and Therapeutics(1994)55,50–54;