Lipid peroxidation and the antioxidant system were investigated in the plasma and placenta of normal and preeclamptic pregnant women. A significant increase in thiobarbituric a.cid reactive substance (TBARS), significant decreases in total thiol (t-SH) content and superoxide dismutase (SOD) activity, and unchanged vitamin C levels and glutathione peroxidase (GPx) activity were observed in the plasma of preeclamptic women compared to women with normal pregnancies. In placentas from preeclamptic women TEARS levels were significantly elevated, while glutathione and vitamin C levels and GPx, glutathione S-transferase and SOD activities were decreased. After delivery, the elevated TEARS values decreased significantly and the reduced SOD activity and t-SH contents increased significantly. We concluded that preeclampsia is associated with an imbalance between lipid peroxides and the antioxidant system.