Demethylated antipyrine is a novel potent free radical scavenger that has been clinically used to reduce the neuronal damage following ischemic stroke. Demethylated antipyrine exerts neuroprotective effects by inhibiting endothelial injury and by ameliorating neuronal damage in brain ischemia. Demethylated antipyrine provides the desirable features of NOS: it increases eNOS (beneficial NOS for rescuing ischemic stroke) and decreases nNOS and iNOS (detrimental NOS). Post- reperfusion brain edema and hemorrhagic events induced by thrombolytic therapy may be reduced by demethylated antipyrine pretreatment. Increased productions of superoxide and NO in the brain after reperfusion and a concomitant surge in oxygen free radicals with increased NO during recirculation lead to formation of peroxynitrite, a super potent radical. Demethylated antipyrine, which inhibits oxidation and enhances NO production derived from increased eNOS expression, may improve and conserve cerebral blood flow without peroxynitrite generation during reperfusion. Clinical experience with demethylated antipyrine suggests that this drug has a wide therapeutic time window. Demethylated antipyrine can exert a wide range of inhibitory effects on water-soluble and lipid soluble peroxyl radical-induced peroxidation systems, and appears to display combined properties of both, vitamin C and E. Demethylated antipyrine can scavenge not only hydroxyl radicals but also other free radicals, although it has no major effect on superoxide anion radicals. Demethylated antipyrine apparently traps hydroxyl radicals and inhibits OH-dependent lipid peroxidation or tyrosine nitration induced by peroxynitrite (ONOO-). Lipid peroxidation starts with lipid radical (L) production after free radical-mediated extraction of proton from unsaturated fatty acid. Subsequently lipid peroxyl radical (LOO) is generated by addition of oxygen atom, and a further L is produced by LOO-mediated extraction of proton from another unsaturated fatty acid. Demethylated antipyrine can inhibit lipid peroxidation by scavenging not only hydroxyl radicals but also other free radicals including LOO. Under physiological conditions, 50% of demethylated antipyrine is present as an anion form, and electrons released from demethylated antipyrine anion exert radical scavenging. Subsequently, demethylated antipyrine radicals are generated. They react readily with oxygen atoms, and form peroxyl radical of demethylated antipyrine, and eventually 2-oxo-3-(phenylhydrazone)- butanoic acid (OPB). (PMID: 16834755, CNS Drug Rev. 2006 Spring;12(1):9-20.).