In fish, estrogens are well known for their involvement in ovarian differentiation and have been shown to be very potent feminizing agents when administrated in vivo during early development. However, the mechanism of action of exogenous estrogens is poorly understood. We report here on the feminizing effects of estrogen treatment on the testicular levels of some steroidogenic enzyme messenger RNAs [mRNAs; cholesterol side-chain cleavage (P450scc), 17-hydroxylase/lyase (P450c17), 3 beta -hydroxysteroid dehydrogenase (3 beta HSD), 11 beta -hydroxylase (P45011 beta), and aromatase (P450aro)] in the rainbow trout, Oncorhynchus mykiss. Treatment was carried out by dietary administration of 17 beta -estradiol (E-2; dosage of 20 mg/kg diet) to a genetically all male population. Steroidogenesis in the differentiating testis was demonstrated to be strongly altered by E-2, as this treatment resulted in considerable decrease in P450c17, 3 beta HSD, and P45011 beta mRNAs after only 10 days of treatment. In contrast, P450scc and P450aro mRNA levels were unaffected by E-2, with P450scc mRNA levels remaining unaltered and P450aro not stimulated by this feminizing estrogen treatment. To better characterize this E-2 effect, the same treatment was applied on postdifferentiating males, and roughly the same expression pattern was detected with a considerable decrease in testicular P450c17, 3 beta HSD, and P45011 beta mRNAs and a significant, but reduced, decrease in P450scc mRNA. In the interrenal, these steroidogenic enzyme mRNAs were not significantly affected by this E-2 treatment, except for a slight, but significant, decrease in P450scc mRNA. These results clearly demonstrate that estrogens have profound effects on testicular steroidogenesis and that they are acting specifically on the testis by decreasing mRNA steady state levels of many steroidogenic enzyme genes. The decrease in P45011 beta mRNA, and thus inhibition of the synthesis of testicular Ii-oxygenated androgens, may be an important step required for the active feminization of these genetic males.