The aim of this work was to study the effect of hormonal sex-reversal treatment during embryonic development of Nile tilapia. XX and XY embryos were incubated in 17 α-methyltestosterone or in 17α-ethynylestradiol during embryonic development. Mean survival rates of embryos 5 days post-hatching ranged from 13.1% to 63.9%. XY embryos incubated in E2 displayed a significant proportion of females (from 50.6 to 79.0%) and XX embryos incubated in MT a significant proportion of males (from 9 to 15%). Our results proved the effectiveness of hormonal sex reversal treatment during embryonic development, as well as precocious sensitivity to hormonal treatments.