The early treatment of male chick embryos with estradiol induces the feminization of their sex tract, i.e. both their gonads and Müllerian tract exhibit female features. The additional treatment of estrogenized male embryos with testosterone propionate antagonizes the effects of estradiol on both gonads and Müllerian ducts. Our data give further support to the view that testosterone and estrogens act respectively as agonist and antagonist modulators of the secretion of the anti-Müllerian hormone.