Gonad histological changes were examined in Japanese medaka exposed to 17beta-estradiol (E2) during early-life stages. Two experiments were conducted at different concentrations of E2 (33.5 and 140.6 ng/L, mean value of measurement) and larvae and juveniles were observed for histological changes in the gonad. Differentiation of ovary and testis in control fish was apparent 12 days post-hatch (dph). At 12 dph, normal testes were observed in male fish that had been exposed to 33.5 ng/L E2, but at 14 and 20 dph, testis-ova was recognized in male fish. Male fish exposed to 140.6 ng/L E2 had testis-ova at 12 dph and gradual transformation to ovary was observed in male fish until 20 dph. In both experiments, the ovarian tissue in testis of male fish exposed to E2 was frequently distributed along the central transverse axis of the gonad, expanding into the transverse axis. The results indicated that 17beta-estradiol can induce testis-ova in male medaka during the larval period and sex-transformation is more frequent at higher (140.6 ng/L) than lower concentrations (33.5 ng/L) of estradiol. The results also demonstrated that testis-ova first appear in the central area of the transverse axis of testis.