We discovered markedly differing catabolism of nicotinamide among rat strains. We compared the catabolism of nicotinamide and also that of the other tryptophan-nicotinamide and water-soluble vitamins among the four strains, Wistar, Sprague-Dawley (SD), August-Copenhagen Irish (ACI) and Fischer 344. The major urinary catabolite of nicotinamide was N(1)-methyl-4-pyridone-3-carboxamide in Wistar, SD and ACI, and N(1)-methylnicotinamide in Fischer rats. This phenomenon was attributed to the enzyme activity involved in the reaction of N(1)-methylnicotinamide to N(1)-methyl-4-pyridone-3-carboxamide being much lower in Fischer than in the other three strains. With the water-soluble vitamins, this specific phenomenon was only observed in the catabolism of vitamin B(6); the urinary catabolite, 4-pyridoxic acid, was much lower too. It was found for the first time that the activities of oxidase were lower in Fischer than in the other strains. This study showed that Wistar, SD, ACI strains had similar water-soluble vitamin metabolism including nicotinamide catabolism.