P eroxynitrite is a potent oxidant formed by the rapid reaction between nitric oxide and superoxide radical (1). The peroxynitrite anion is relatively stable, but it can rapidly protonate to peroxynitrous acid, an unstable species, which decomposes with a half-life of about one second at pH 7.4 (1,2). Peroxynitrite can serve as a precursor for other potent reactive species, including nitrogen dioxide, nitronium ion, and an intermediate with hydroxyl radicallike reactivity toward a variety of biological molecules (2). Propofol(2,6-diisopropylphenol) has been used extensively as a rapidly acting intravenous anesthetic with a high therapeutic index for both inducing and maintaining general anesthesia (3). Recent in vitro studies have demonstrated that propofol has potent antioxidant properties very similar to those of a-tocopherol (4-6). Propofol inhibits lipid peroxidation induced by oxidative stress in the rat liver microsomes (5, 6), mitochondria, and brain synaptosomes (6). Although propofol reacts primarily with organic or organoperoxyl radical species (7), its effect on peroxynitrite is not known. Chemiluminescence has been widely used for measuring the production of free radicals and reactive oxygen metabolites from enzyme, cell, or organ systems (8, 9). In the present study, we used the chemiluminescence method for the assessment of the scavenging activity of propofol against peroxynitrite.