In the rat, testosterone (T) in the neonatal period plays an important role in sexual differentiation and there is a serum T surge in male rats 2 hours after birth. Pregnant female rats were exposed to various doses of bisphenol A (BPA) from gestational day 1 (GD1) through 2 hours after parturition. About half of the BPA-exposed and control dams were subjected to cesarean section on GD22. The male fetuses on GD22 were immediately sacrificed and blood was collected. The other half of the BPA-treated and control dams delivered at GD23 (parturition day). The male pups were sacrificed 2 hours after birth. Serum T concentration was determined by radioimmunoassay (RIA). The BPA concentration in the fetal serum on GD22 increased inversely to the T levels in the serum. The T concentration in the pups' serum 2 hours after birth decreased inversely to the BPA concentration in the serum. However, there were no differences in the serum T concentration among the various doses of BPA. These results suggest that exposure to BPA in utero inhibits the T surge in the neonatal period and we speculate that exposure to BPA in utero disrupts the endocrine environment in the neonatal male.