Endothelial cells and activated phagocytes produce both nitric oxide ((̇)No) and superoxide (O2(̇-)), which react to form peroxynitrite. Peroxynitrite has been suggested to be directly cytotoxic and also to decompose into other toxic species. In order to understand the consequences of peroxynitrite generation in vivo, we examined its reaction with human blood plasma. Peroxynitrite decreased the total peroxyl-trapping capacity of plasma. In terms of specific antioxidants, addition of peroxynitrite to plasma leads to rapid oxidation of ascorbic acid, uric acid and plasma SH groups. The oxidation of plasma SH groups was enhanced in dialysed plasma and returned to control levels by the addition of physiological levels of bicarbonate. Evidence was found for formation of nitro-adducts to aromatic side chains in plasma proteins by peroxynitrite. Peroxynitrite also leads to depletion of ubiquinol and formation of traces of lipid hydroperoxides in plasma, although α-tocopherol levels were only slightly decreased. Peroxynitrite formation in human body fluids is likely to cause antioxidant depletion and oxidative damage.