In fish, estradiol-17 beta(E2) regulates various reproductive processes by acting through estrogen receptors (ERs). Here, we cloned three ER subtypes from medaka and examined their developmental expression in the gonads and liver of genetically females and males from embryos to adults. During embryogenesis, marked increases in the expression of ERN, but not either ER alpha or ER beta 1, were found in genetically female embryos during sex differentiation. E2 treatment induced marked up-regulation of ER beta 2 expression in genetically male embryos. In adult ovaries, ERa levels were high in follicles (granulosa cells) during oocyte growth. In the testis, ER beta 1 expression exhibited a distinct peak at 10 days post hatching (dph). In the liver, very high levels of ER beta 2 were found in both females and males throughout the sampling period with significantly higher levels in females during 30-50 dph. These findings suggest each action of E2 to be mediated by different types of ERs. (C) 2010 Elsevier Ireland Ltd. All rights reserved.