It has been suggested that oxidative stress may cause endothelial dysfunction and that endothelial dysfunction may lead to hypertension by reduced release of vasodilating agents such as nitric oxide (NO). In this study, we investigated the relationship between serum NO and lipid peroxides in preeclamptic and normal pregnant women before and after delivery. Plasma from women with preeclampsia had significantly lower nitrate/nitrite concentrations and significantly higher lipid peroxide levels than normal pregnant women before the delivery. Lipid peroxide levels were significantly elevated in preeclamptic placenta. After delivery in the preeclamptic group the plasma concentration of nitrate/nitrite was increased and plasma thiobarbituric acid reactive substance levels decreased, while these parameters remained unchanged in the normal pregnants women. These results indicate that high levels of lipid peroxides in the circulation may be the cause of lowered NO synthesis and hypertension observed in preeclamptic women. Copyright (C) 1999 S. Karger AG, Basel.