Effect of prenatal exposure to methylmercury (MeHg) on the glutathione (G-SH) levels and lipid peroxidation in the fetal brain was examined. Pregnant ICR mice mere injected with 3 mgHg/kg of:MeHg on gestational day 12, 13 and 14 (G12-14). On the G14 or G17, the fetal brains were removed and their GSH levels and thiobarbituric acid-reactive substances (TBARS) levels were determined. On the G17, GSH level of MeHg-treated fetal brain was significantly higher than that of the control brain; the TEARS level showed the similar trend but the difference was not significant. These results indicated that the prenatal MeHg treatment disturbed the normal GSH level in the fetal brain and warranted further investigation on the significance of this GSH perturbation.