Ethanol markedly inhibits the biosynthesis of testosterone in the male of several species. Since several in vitro studies have suggested that ethanol per se is not a gonadal toxin and that it must be metabolized to exert its effects, we examined this possibility under in vivo conditions in the present studies. We found that the administration of the alcohol dehydrogenase inhibitor, pyrazole, to adult male rats significantly elevated blood ethanol levels. However, rather than resulting in a potentiation of the effects of ethanol on testicular steroidogenesis, pyrazole-induced elevations in blood ethanol concentrations produced a significant attenuation of ethanol's effects. In view of these observations, it is difficult to maintain that ethanol itself is responsible for inhibiting the production of testosterone. On the contrary, our results may provide the first in vivo support for the hypothesis that ethanol must be metabolized to exert its effects on testicular steroidogenesis.