[show abstract][hide abstract] ABSTRACT: There is now compelling evidence that inappropriate exposure to estrogen during fetal or neonatal life could affect adult reproductive functions because the testis is sensitive to estrogens during specific periods of its development. Therefore, we investigated the effects of exogenous estrogens on gametogenesis and steroidogenesis during fetal and neonatal testicular development in the rat. We used in vitro systems, organ cultures, and dispersed testicular cell cultures, which allow the development of fetal and neonatal germ cells (gonocytes) and Leydig cells. Exogenous estrogens inhibited testosterone production in dispersed testicular cell cultures throughout fetal life, but this inhibition was observed only in the early fetal stages in organ culture. By using an aromatase inhibitor (letrozole, Novartis Pharma AG), we showed that the inhibitory effect of exogenous estrogens on testosterone production is masked in the whole testis at later stages (20.5 days postconception) due essentially to local production of estrogens. In both systems, additions of high concentrations (10(-6) M) of 17beta-estradiol or diethylstilbestrol decreased the number of gonocytes during the first fetal proliferative period but not during the neonatal period. Letrozole was without effect, suggesting that the aging-related loss of responsiveness of gonocytes is not due to any aromatase activity in the gonocytes.