To induce sex reversal of male to female, freshly-fertilized eggs of the S-rR strain medaka (Oryzias latipes) were immersed in saline containing estradiol-17β (E₂) in different concentrations for various durations until hatching. Results of the present experiment showed that the immersion duration in 1 μg/ml E₂ to induce 100% reversal of sex differentiation in the genotypic males was enough only for one day (24 hr) post-fertilization (dpf) and that treatment with E₂ for 1 dpf resulted in a dose-dependent manner with the maximum sex reversal of 100% at 1 μg/ml. To ascertain early developmental periods efficacious for inducing sex reversal, additional brief immersion treatments of eggs with E₂ were further performed individually for four different early developmental periods (Stages 4–9, 10–12, 13–15 and 16–18) within 1 dpf. As a result, induction of sex reversal was observed in all these short immersion periods without any restricted efficacy. Between both experimental and control groups treated with or without E₂ for 1 dpf, differences in the number of germ cells in a gonad were compared in newly-hatched fry. It was found that gonads of the genotypic males (XY) treated with E₂ revealed the female type which contained many germ cells with much dividing activity. These data suggest that a possible switch mechanism that exogenous E₂ could trigger to change the genetic cascades involved in sex determination upon fertilization exists in early developmental stages.