Metabolism of progesterone in ovaries of prepubertal rats has been investigated by incubating intact ovaries from immature rats in Krebs-Ringer bicarbonate buffer containing 10-6 M progesterone-4-[14C] and isolating the radioactive metabolites formed after varying periods of incubation. Androsterone was the most abundant metabolite produced by ovaries from 28-30-day-old rats. Luteinizing hormone (LH) injected 0.5-4 h before autopsy, or added directly to incubation media, resulted in markedly decreased formation of androsterone with a concomitant proportional increase in production of 3α-OH-5α-pregnane-20-one (3α-OH-5α-P). Production of other androgens (androstenedione, testosterone, 5α-androstane-3α,17β-diol), which accumulated in smaller amounts than androsterone, was also inhibited by LH. Prolactin administration for 6 days before autopsy resulted in only minor alterations of progesterone metabolism, causing small increases in accumulation of 5α-pregnane-3,20-dione (5α-P) and 3α-OH-5α-P in the absence of LH and slightly enhancing the inhibitory effects of LH on formation of androstenedione and androsterone.