Gender difference in human bisphenol A (BPA) concentrations was revealed by determining serum BPA. We studied the serum concentrations and the metabolism of BPA in rats by an HPLC system. Rat serum BPA concentrations were significantly higher in males (24.9 +/- 7.38 ng/ml, P = 0.026, n = 10) than in females (8.27 +/- 3.11 ng/ml, n = 10), as in humans. The resultant enzyme reaction products of BPA glucuronidation in the rat liver microsomes fraction were analyzed by an HPLC system. The ratio of BPA glucuronidation in the microsome reaction was significantly higher (P = 0.015) in female than in male rats. The mRNA expression of UDP-glucuronosyltransferase 2B1 (UGT2B1), an isoform of UGT related to BPA glucuronidation, in the rat liver was analyzed by a real-time quantitative RT-PCR. The relative expression level of UGT2B1 mRNA was significantly higher (P < 0.001) in female than in male rat livers. The gender difference in serum BPA concentrations may be explained by the difference in clearance based on the UGT activities. (C) 2004 Elsevier Inc. All rights reserved.