The present study was performed to investigate in vitro the onset of steroidogenesis and the responsiveness to LH in rat fetal testes. The male gonads explanted on days 12.5, 13.5, and 14.5 of gestation in M199 produced testosterone from 15.5 days as is the case in vivo dcAMP (1 mM) induced an anticipated steroidogenesis on day 14.5 with secretions of testosterone (0.026 +/- 0.003 ng/gonad/24 h) and progesterone (0.078 +/- 0.005 ng/gonad/24 h), whereas LH (100 ng/ml) has no effect. Antiandrogens such as aminoglutethimide (2 mM), cyproterone acetate (1 microgram/ml), and hydroxyflutamide (1 microgram/ml) could not delay the responsiveness to LH on day 15.5. An anticipated production of testosterone on day 14.5 in presence of DHA (200 ng/ml) could not induce functional LH receptors. It would appear that: (a) the onset of testosterone production occurs without extrinsic stimulatory factors; (b) dcAMP initiates an early steroidogenesis; (c) the onset of functional receptors is likely free of the androgenic environment.