Estrogens exert various biological effects by acting through their native receptors, two of which have been identified to date: estrogen receptors α (ERα) and β (ERβ). In this study we examined the expression and cellular localization of ERα and ERβ in various human fetal tissues by semiquantitative RT-PCR (13 and 20 gestational weeks) and immunohistochemistry (13, 20, and 38 gestational weeks), respectively, to study the possible effects of estrogens on human fetal tissues during development. Relatively high levels of ERβ expression were detected in various human fetal tissues, whereas those tissues expressing ERβ had markedly lower levels of ERα expression. ERβ messenger ribonucleic acid expression was especially high in the adrenal gland. ERβ-immunoreactive protein was localized to the definitive zone, but not in the fetal zone, of the adrenal cortex. Although low levels of ERβ messenger ribonucleic acid were present in the brain, heart, lung, and kidney, ERβ immunoreactivity was not detected in these tissues. These results suggest that the effects of estrogens in these tissues are predominantly mediated through ERβ. ERβ immunoreactivity was detected in Sertoli cells and spermatogonia in the male reproductive tract and in germ cells in the fetal testis and epididymis. In the female reproductive tract, both ERα and ERβ were immunopositive in epithelium of the oviduct. The results of the present study have demonstrated the possible sites for estrogenic action in the human fetus and suggest that the effects of estrogen via ERβ may play important roles in human fetal development, especially in the definitive zone of the adrenal cortex, and in the reproductive tissues of the developing fetus.