Niacin is biosynthesized from tryptophan and its ability is enough to sustain the demand of niacin animal or human body. Therefore, niacin itself does not need to take. Vitamin B groups and minerals are required in the conversion of niacin from tryptophan. Thus, we investigated the effects of feeding with a thiamin-free, riboflavin-free, pyridoxine-free, or mineral-deficient diet on the conversion ratio of tryptophan to niacin in rats. In the thiamin-free experiment, the ratio was higher in the thiamin-free group than in the control. In riboflavin-deficient rats, the conversion ratio was not affected. In the pyridoxine-free experiment, the ratio was lower in the pyridoxine-free group than in the control. In the mineral-deficient experiment, the sum of the urinary excretion of nicotinamide and its metabolites was lower in a deficient group than in the control. Accordingly, deficiencies of pyridoxine and/or minerals directly caused niacin deficiency, but thiamin- and riboflavin-deficiencies did not.