In normal pregnancy the vasodilating actions of prostacyclin and the antioxidant activity of vitamin E are important for normal physiologic function. Thromboxane and lipid peroxides oppose these actions by promoting vasoconstriction and peroxidation reactions, respectively. An imbalance between thromboxane and prostacyclin and between lipid peroxides and antioxidant activity is implicated in pathologic states such as preeclampsia. We hypothesized that in normal pregnancy there would be a balance in the ratios of prostacyclin to thromboxane and of vitamin E to lipid peroxides that would favor prostacyclin and vitamin E. Blood samples were collected from normally pregnant women throughout gestation and analyzed for prostacyclin, thromboxane, vitamin E, and lipid peroxides. Serum levels of lipid peroxides remained relatively stable throughout gestation, but the levels of vitamin E progressively increased. Plasma levels of prostacyclin progressively increased with advancing gestation, whereas levels of thromboxane progressively decreased. Therefore the ratios of both prostacyclin/thromboxane, and vitamin E/lipid peroxides progressively increased during pregnancy. The increase in the ratios was highly correlated, r = 0.94. We conclude that the changes in the maternal concentrations of these compounds and the progressive increase in the ratios of prostacyclin/thromboxane and vitamin E/lipid peroxides suggest that the vasodilating actions of prostacyclin and the antioxidant activity of vitamin E are progressively favored with advancing gestation in normally pregnant women.