Xenoestrogens, such as bisphenol A and diethylstilbestrol, are glucuronidated by an isoform of UDP-glucuronosyltransferase named UGT2B1 in the livers of adult male rats. In this study, we found that nonylphenol and octylphenol are also conjugated with glucuronic acid by adult rat liver microsomes. Although UDP-glucuronosyltransferase activities toward these xenoestrogens were not detected in the fetal rat liver, a linear increase in enzymatic activities during neonatal development was observed. At 3 weeks after birth, the activities had reached the same level as that of adult rats. The protein and mRNA contents of UGT2B1 also were not detected in the fetal rat liver, but a developmental increase in newborn rat liver was detected by Western and Northern blotting analysis. Additionally, rat hepatic microsomal UDP-glucuronosyltransferase activities toward these xenoestrogens were reduced by about half during pregnancy of mother rats. The results suggest that the reproductive organs of fetal and early-stage neonatal rats, which are sensitive to sex hormones, face a high risk of exposure to free active xenoestrogens.