It was demonstrated by human experiments that the fecal and urinary niacin markedly increased with the change of an ordinary diet to a vegetable one. The increase was more marked when cellulose was added to the vegetable diet. On the contrary, the fecal and urinary niacin decreased after the diet was switched from an ordinary diet to a meat one. The addition of cellulose to a meat diet however caused a definite rise in the amounts excreted. Furthermore, it was demonstrated that both N^1-methyl-nicotinamide and N^1-methyl-3-carboxylamide-6-pyridone were excreted parallel with niacin and the niacin synthesized amounted to 55-58 mg per day if all these metabolites in urine were taken into consideration.