In order to investigate the age-specific effect of estrogen receptor α (Erα) in testis and epididymis of mandarin voles (Microtus mandarinus), the expression of Erα from five age groups [1 (neonatal), 10, 25, 45, and 60 (adult) days of age] was observed using immunohistochemistry. There were several kinds of Erα-immunoreactive cells in the testes and epididymis of normal voles, such as Leydig cells, spermatogenic cells, as well as myoid cells and epithelial cells of the epididymis. The distributions and staining intensity of Erα-immunoreactive cells changed greatly with age. Statistics showed that: the expression of Erα in pros-permatogonium cells and gonocyte was weak in the neonatal vole. The expression of Erα in spermatids appeared at postnatal day 25, as well as day 45 of age. The expression of Erα was detected in every developmental stage of germ cell in the adult vole. The expression of Erα in Leydig cells appeared in the neonatal vole followed by a decrease in intensity at postnatal day but increasing at postnatal day 25. The expression intensities of Erα in Leydig cells peaked at day 45 but decreased in the adult. The expression of Erα in epithelial cells was found in the neonatal vole and was detected in myoid cells at postnatal days 10 and 25. In the epididymes of 45 day old and adult voles, myoid cells and epithelial cells expressed Erα positively. Those results imply a molecular mechanism of Erα action. Specifically, Erα may affect the secretion of androgen in Leydig cells as special receptors during postnatal development and spermatogenesis of mandarin voles.