In fish, according to Yamamoto's model, androgens would drive testis differentiation and estrogens ovarian differentiation. In order to study the implication of steroid enzymes in rainbow trout gonadal differentiation, we examined the expression of some steroid enzyme genes during natural differentiation (cholesterol side chain cleavage = P450scc, 17-hydroxylase/lyase = P450c17, 3 beta -hydroxysteroid dehydrogenase = 3 beta HSD) and androgen-induced differentiation (P450scc, P450c17, 3 beta HSD, aromatase = P450aro, and 11 beta -hydroxylase = P45011 beta). Expressions of P450scc, 3 beta HSD, and P450c17 were all detected in male and female gonads at 55 days post-fertilization (dpf), i.e., two weeks before histological differentiation. There were no differences in their expression level respective to the sex. The androgen treatment was carried out by administration of 11 beta -hydroxyandrostenedione (11 beta OH Delta4) in genetic all-female populations and the resulting sex ratios were found to be 100% male even at a low dosage of 1 mg/kg of food. Following 11 beta OH Delta4 treatment, only the expression of P450c17 was found to be sustained when compared with the female untreated control. In contrast, P450scc was clearly up-regulated and 3 beta HSD and P450aro down-regulated by the androgen treatment. P45011 beta gene expression remained low in gonads of androgen-treated females, as it did in control untreated females. These results together demonstrate that steroidogenesis in rainbow trout is potentially active in predifferentiating gonads of both sexes, and that one of the masculinizing actions of androgens in the species may be to down-regulate the female-specific gonadal P450aro gene expression. However, in vivo androgen treatment in genetic females does not induce the same pattern of steroid gene expression as in genetic males. These data suggest that exogenous androgens might induce a male differentiation process with P450aro inhibition being one of the steps required. However, this process would not involve endogenously produced 11-oxygenated androgens. J. Exp. Zool. 290:558-566, 2001. (C) 2001 Wiley-Liss, Inc.