Two groups of Japanese medaka (Oryzias latipes) were exposed to 17beta-estradiol (E2: 150ng/L, nominal concentration) for either a short-term exposure (STE: 0-31 days after fertilization (daf); egg-larval period) or a long-term exposure period (LTE: 0-81 daf; egg-adult period) and their subsequent spawning performance was compared in terms of fecundity, spawning time, and fertility. Most genetic males were transformed to phenotypic females by E2 following both short-term and long-term exposure, but spawning performance and gonad somatic index (GSI) of sex-transformed females (XY females) following long-term exposure were lower than those of sex-transformed females following short-term exposure and those of normal females (XX) in the control group. Sex-transformed females in the STE group and normal females possessed mature ovary, whereas most of the sex-transformed females in the LTE group possessed immature ovary, with most oocytes being in the pre-vitellogenic phase. Moreover, the chromosome types of first filial generation delivered from sex-transformed female in STE group composed with 51.9% as XY, 18.5% as YY, and 29.6% as XX. From these results, it seems that exposure to E2 until the end of the larval period produces sex-transformed medaka with high reproductive ability, similar to normal females, but longer exposure to E2 may inhibit sexual maturation in the sex-transformed female.