Serum concentrations of testosterone plus dihydrotestosterone (T-DHT), 5 alpha-androstane-3 alpha, 17 beta-diol (Diol) and androsterone were measured during sexual maturation in male rats. Diol concentrations of 1 to 2.25 ng/ml were found in animals 10-90 days of age with no significant changes. Diol was the major androgen (5.75-8 times T-DHT) from age 20-40 days. Androsterone rose to 1.25 ng/ml at 25 days of age and declined to values of greater than 0.5 ng/ml from age 30-90 days. Testosterone-DHT levels were 1 ng/ml or less from 10-40 days of age, and then rose to a peak at 60 days. The ratio of Diol toT-DHT was significantly elevated from age 20-35 days, indicating that Diol is the major androgen in circulation at this time. Acute treatment of 33 day old rats with LH, but not with FSH, resulted in a dose-dependent increase in serum T-DHT, Diol, and androsterone. The dose-response and time course of response for the three steriods were nearly identical. Changes in testes capacity to secrete androgens were assessed 25, 33, 40 and 60 days of age by administering a maximum dose of LH before blood collection. Maximum response (sum of androsterone, Diol and T-DHT) occurred at 40 days with no further increase at 60 days.