Two experiments investigated the role of estrogens in the activation and sexual differentiation of appetitive sexual behavior (ASB) in Japanese quail (Coturnix japonica) as measured by a learned social proximity response. Injection of the aromatase inhibitor R767 13 in castrated, testosterone (T)-treated male quail completely suppressed ASB, confirming that, like consummatory sexual behavior, ASB is mediated by T aromatization. ASB is not observed in female quail, even if they are treated with T as adults. The role of embryonic estrogens in the sexual differentiation of ASB was tested by blocking estrogen synthesis in ovo. Control male and T-treated female quail deprived of estrogens during embryonic life learned the social proximity response used to assess ASB, whereas control female quail did not, despite the presence of high T. Thus, ASB is demasculinized by the action of embryonic estrogens during ontogeny as is consummatory behavior.