Nitric oxide and superoxide rapidly combine to form a toxic reaction product, peroxynitrite anion (ONOO? ) (1 ,2 ). The oxidant reactivity of peroxynitrite is mediated by an intermediate with the biological activity of the hydroxyl radical. However, this product does not appear to be the hydroxyl radical per se , but peroxynitrous acid (ONOOH) or its activated isomer (ONOOH*) (2 ). Peroxynitrite readily reacts with proteins, lipids, and DNA under conditions of inflammation.