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Objective: To assess the effect of endogenous estrogens on the bioavailability of nitric oxide (.NO) and in the formation of lipid peroxidation products in pre- and postmenopausal women., Methods: NOx and S-nitrosothiols were determined by gaseous phase chemiluminescence, nitrotyrosine was determined by ELISA, COx (cholesterol oxides) by gas chromatography, and cholesteryl linoleate hydroperoxides (CE18:2-OOH), trilinolein (TG18:2-OOH), and phospholipids (PC-OOH) by HPLC in samples of plasma., Results: The concentrations of NOx, nitrotyrosine, COx, CE18:2-OOH, and PC-OOH were higher in the postmenopausal period (33.8+/-22.3 microL; 230+/-130 nM; 55+/-19 ng/microL; 17+/-8.7 nM; 2775+/-460 nM, respectively) as compared with those in the premenopausal period (21.1+/-7.3 microM; 114+/-41 nM; 31+/-13 ng/microL; 6+/-1.4 nM; 1635+/-373 nM). In contrast, the concentration of S-nitrosothiols was lower in the postmenopausal period (91+/-55 nM) as compared with that in the premenopausal p in the premenopausal period (237+/-197 nM)., Conclusion: In the postmenopausal period, an increase in nitrotyrosine and a reduction of S-nitrosothiol formation, as well as an increase of COx, CE18:2-OOH and PC-OOH formation occurs. Therefore, NO inactivation and the increase in lipid peroxidation may contribute to endothelial dysfunction and to the greater risk for atherosclerosis in postmenopausal women.