Exogenous sex-steroid hormones and Letrozole(LE),a potently synthetic nonsteroidal aromatase inhibitor(AI),interfere in the committed step of the endogenous estrogens synthesis.In the present study,the efficacy of 17α-Methyltestosterone(MT)and Letrozole(LE)on gonad steroidogenic enzyme gene expression was evaluated in Nile tilapia(Oreochromis niloticus).The genetically female tilapia were treated with 17αMethyltestosterone and Letrozole(at a dose of 10 mg/kg).Expressions of cytochrome P450 aromatase(P450arom),11β-hydroxysteroid dehydrogenase type 2(11β-HSD₂)and cytochrome P450 cholesterol-side-chain-cleavage(P450scc)were examined in the gonads after injection treatment for 12 hours,24 hours,48 hours,7 days,14 days and 21 days,respectively.The results showed that the levels of P450arom,11β-HSD₂ and P450scc mRNA were decreased rapidly compared to the untreated group firstly,and then the mRNA levels of the three genes increased to the levels of pretreatment.In the MT-treated group,the mRNA levels of P450scc were first decreased,followed by P450arom and 11β-HSD₂.While in the LE-treated group,the mRNA levels of P450arom and P450scc were firsted decreased,followed by 11β-HSD₂.These results indicated that exogenous androgen or aromatase inhibitor treatment suppresses the expressions of key steroidogenic enzyme genes including P450arom,11β-HSD₂ and P450scc in the female tilapia.Furthermore,it shows that MT may indirectly inhibit the expression of P450arom,while LE directly inhibits the expression of P450arom,and further influences the sex-inversion of fish.The present study suggests that besides steroid hormones,nonsteroidal compounds,such as aromatase inhibitors,have potential for production of monosex population in tilapia.