The conversion of N^1-methylnicotinamide (MNA) and trigonelline (TRG) to niacin and their toxicity in rats were investigated. The conversion of MNA into nicotinamide in rats did not seem to occur since the niacin effect was not observed when MNA was administered with tryptophan-imbalanced-niacin-free diet to weanling rats. When a large amount of MNA or TRG was administered to weanling rats, no growth retardation was observed. All of the TRG taken was excreted unchanged into urine. On the other hand, 10% of MNA taken was metabolized to N^1-methyl-2-pyridone-5-carboxamide and 70% of MNA was excreted unchanged. Even when a large amount of TRG or MNA was administered to rats, urinary excretion of niacin did not increase. From these results, it is concluded that the conversion of MNA and TRG into niacin does not occur and MNA and TRG are inert and very safe compounds in rats.