The chemical oxidative conversion of creatinine (Cr) into methylguanidine (MG) has been followed by 1H-NMR and HPLC. By using active oxygen species generated by Fenton's reagent (Fe2+ and H2O2) or the similar reagent (Fe3+ and H2O2), creatol (CTL), a metabolite newly isolated from the urine of uremic patients, and creatones A and B were experimentally detected in the reaction mixture and implicated as successive intermediates in the pathway from CR to MG. An alternative oxidation of Cr to demethylcreatinine (glycocyamidine) was also observed. The importance of CTL in this in vitro oxidation mechanism is discussed.