Plasma levels of guanidine (G) are reported to be increased in uremic patients and are synthesized from various guanidino compounds via a chemical reaction involving the hydroxyl radical in vitro. To identify both the metabolic precursor and the synthesizing organ of G, we investigated the concentrations of G in various organs of rats administered several guanidino compounds and we attempted to synthesize G biologically using isolated rat hepatocytes or perfused rat liver. In addition, we investigated the effect of the peroxidative state on the G synthesis in isolated hepatocytes using various reagents which alter this condition. Results show that the concentration of G increased in the kidney, liver and muscle following the administration of L-canavanine. In addition, G increased in the kidney at 90 min after the administration of guanidinoacetic acid (GAA). Moreover, G is synthesized from L-canavanine in isolated rat hepatocytes and perfused rat liver, and G synthesis in hepatocytes is partially inhibited by the addition of superoxide dismutase, catalase, glutathione or ethylenediaminetetraacetic acid. These results suggest that L-canavanine is possibly a biological precursor and GAA is an endogenous precursor of G. Furthermore, it is suggested that these reactions are closely related to the peroxidative state.