Treatment of hypophysectomized immature male rats with luteinizing hormone (LH) greatly increased the metabolism of both 4-[14C]progesterone and 4-[14C]testosterone by testicular homogenates. Prolactin, either alone or in combination with LH, did not influence the metabolism of either substrate. Progesterone was metabolized to 17α-hydroxyprogesterone, androstenedione, 5α-pregnan-3,20-dione, 3α-hydroxy-5α-pregnan-20-one, and 3β-hydroxy-5α-pregnan-20-one. Testosterone was metabolized to dihydrotestosterone and 5α-androstan-3α,17β-diol. On the basis of these observations it is suggested that LH stimulated the 5α-reductase(s) of the immature rat testis. Testis homogenates from immature rats with intact pituitaries were incubated with 4-[14C]3α-hydroxy-5α-pregnan-20-one. Rapid conversion to androsterone was observed, with the formation of a compound chromatographically identical with 3α, 17α-dihydroxy-5α-pregnan-20-one as an apparent intermediate. These findings demonstrate the ability of the rat testes to form androsterone from progesterone by a pathway that does not involve testosterone.