In view of the evidence that there may be an effect of high concentrations of oestradiol on testicular steroidogenic function, we have investigated the effect of this steroid on [3H]uridine incorporation into RNA by testicular cells. Our results have shown that oestradiol in vitro induced a marked dose-dependent inhibition of RNA synthesis by purified Leydig cells. The concentrations of oestradiol tested varied from 2 to 40 mumol/l; these concentrations also impaired net testosterone synthesis in vitro after human chorionic gonadotrophin (hCG) stimulation. Under the effect of oestradiol, the kinetics of [3H]nucleoside incorporation into RNA were impaired early and the inhibition of RNA synthesis was specific for oestrogenic compounds. It was concluded that, in Leydig cells, oestradiol, in addition to its known inhibitory action on the response of testosterone to hCG, triggers a more extensive response that also includes RNA synthesis in vitro.