Sex reversal effects of 17 beta-estradiol (E₂), diethylstilbestrol (DES) and ethynylestradiol (EE₂) on male gonads in F₁ (AWE×WE) Japanese quail (Corturnix japonica) embryos were comparatively evaluated in a newly developed in vivo screening model called as the sex reversal test. Male and female offspring of F₁ (AWE×WE) Japanese quail exhibit exactly wild and albino plumage colors, respectively, ruled by a criss-cross inheritance. The natural and synthetic estrogens were injected into egg white just before the incubation. At 16 days of incubation, embryos were subjected by a complete necropsy and their gonads were grossly observed and examined histopathologically and morphometrically. Grossly, genetic sex confirmed by plumage colors coincided completely with external sex phenotype of the gonads in all embryos of the control group and E₂ and DES-treated groups. However, several male embryos with wild plumage in the EE₂ 2000ng group possessed an ovary-like gonad in the left side and a vestigial right gonad. Histopathologically, E₂, DES and EE₂ exposures induced a dose-dependent sex reversal effect, i.e. ovotestis development, in the left testis. The left testes showing an ovary-like morphology in the EE₂ 2000ng group consisted of the most of area replaced with ovarian tissue and the small area of remaining testicular cords. The incidence and morphometric analysis of the ovotestis revealed that the order of potency of sex reversal effect in Japanese quail embryos was EE₂>DES>E₂. E₂, DES and EE₂ exposures induced no noticeable changes in the ovaries of any embryos. The present study suggests that the sex reversal test using F₁ (AWE×WE) Japanese quail embryo is possible to evaluate feminization effects of endocrine disrupting chemicals with estrogenic activities in avian male embryos.