Exogenous estradiol benzoate (EB) or estradiol-17β (E2) caused dose-dependent gonadal feminization of slider turtle (Trachemys scripta) embryos incubated at a male-producing temperature (26°), suggesting that sex reversal requires a threshold dosage of these hormones. Even at dosages resulting in mixtures of males and females, nearly all hatchlings had normal-appearing ovaries or testes. Only 7 of 241 hatchlings had gonads that had not differentiated fully into ovaries or testes. Thus, with rare exception, estrogens exerted an "all or none" effect. The transport of hormone into the embryo varied with mode of administration and E2 was more effective than was EB at the lowest dosage used. These studies suggest either that exogenous estrogen is capable of coordinating cortical/medullary development in the gonad so that intersexes are prevented or that estrogen acts "upstream" of the developmental processes responsible for coordinating gonadal development. © 1991.