Dihydrotestosterone propionate (DHTP) administered to intact, 25-day-old rats over a period of 6 days causes a significant decrease in testicular weight. Concurrent administration of gonadotrophic hormone prevents this decrease in testicular weight (Feder, 1971). Similarly, Beyer, Moralí & Cruz (1971) demonstrated that DHTP blocked ovarian compensatory hypertrophy in female rats. These findings and earlier reports (Bottomley & Folley, 1938; Dorfman & Kind, 1966) suggested that dihydrotestosterone suppressed secretion of gonadotrophic hormones. However, direct measurements of plasma concentrations of the gonadotrophins after DHTP treatment of young rats were unavailable. This report offers quantitative data on systemic plasma concentrations of luteinizing hormone (LH) in young male rats treated with DHTP. A previously described radioimmunoassay method was used for LH measurement (Niswender, Midgley, Monroe & Reichert, 1968; Naftolin & Corker, 1971). In one experiment intact Sprague—Dawley male rats were injected daily with 125 μg DHTP, 375 μg DHTP, 125 μg testosterone propionate (TP)