The histological features, steroidogenic activities (aromatase, I I P-hydroxylase) and gene expressions (cyp19A1, cyp11b1, amh, ddx4, gcl, fst) of trout embryonic gonads have been compared before and after 7 days of in vitro culture at 12 degrees C in three different media. MCDB 105 gave the best results and was used to analyse the effect of 50 nM 17 beta-estradiol on testicular cyp11b1 expression. A significant six-fold decrease has been observed for cyp11b1, while amh, ddx4 and gcl did not change.