Methylguanidine (MG), a toxin reported in uremia, is thought to be a product of creatinine oxidation. This study is designed to demonstrate the role of active oxygen in the oxidation of creatinine under conditions compatible with those found in uremia. MG synthesis is moderately stimulated by the superoxide radical derived from 3 mM hypoxanthine and 0.015 units/ml xanthine oxidase and inhibited by the addition of superoxide dismutase. This is increased markedly by the addition of 0.05% hydrogen peroxide and augmented to about 56,000 times the control rate in the presence of hydroxyl radicals derived from the reaction of 10 mM FeSO4 and 0.05% hydrogen peroxide. In addition, MG synthesis is inhibited by the addition of sorbitol, lactulose or ethanol, the scavengers of hydroxyl radicals. These results indicate that creatinine can be oxidized to MG by various species of active oxygen and that one of the mechanisms of MG synthesis is such oxidation. MG, therefore, may be a useful indicator of peroxidation in vivo.