Bisphenol A (BPA,at dose of 5,50,or 100 mg /kg) was given daily by gavage to female mice (Mus musculus) during lactation,and thus the pre-weaning baby mice were exposed to BPA from maternal milk. The effects of BPA on testis development and the expression of estrogen receptor (ER) α and proliferating cell nuclear antigen (PCNA) in testes were studied. In addition,the serum level of estrogen and some antioxidant enzymes were detected. Results showed that serum concentration of estrogen in mature male mice was higher than that in the vehicle-treated group. The exposed mice had a significant decrease in the activities of glutathione peroxidase (GSH-Px) and the total superoxide dismutase (SOD). Morphological studies revealed that testis development was inhibited by BPA with germ cells in disordered arrangement. BPA reduced the number of caudal epididymal sperm and increased the number of abnormal sperm significantly. Immunohistochemistry analysis revealed that there were two different patterns of ERα expression,presence only in the nuclei of Sertoli cells or in the nuclei of spermatocytes except for Sertoli cells. PCNA-positive cells were increased in the latter ERα expression pattern. The average integrated optical density (IOD) of ERα-positive single germ cell in BPA-treated groups was significantly higher than that in the vehicle-treated group. The expression of PCNA in primary spermatocytes was also increased in BPA-treated groups.