Different mechanisms of genetic sex determination in birds are considered. Special emphasis is placed on the influence of external factors (reduction of estrogen synthesis, inactivation of DNA methylases) on expression of key sex determining genes. The possibility of sex inversion in domestic chicken by aromatase inhibition, genomic DNA demethylation, and the effect of estrogen analogs on estrogen receptors was demonstrated. The results of these effects were analyzed and their effect on the development of gonads in one-day embryos was demonstrated. A suggestion on earlier aromatase activity in chicken embryos, weak effect of DNA demethylation with 5-azacytidine, and different mechanisms of the effect of estrogens and their analogs (tamoxifen) on the gonad development in chicken was made.