Summary(1) Four subjects subsisted for a period of 3 months on a diet of purified vitamin-free foods furnishing a daily intake of 1.5-2.0 mg nicotinamide. (2) There were no symptoms or signs of nicotinic acid deficiency observed during this period.(3) The urinary output expressed as N-methylnicotinamide chloride showed no reduction suggestive of depletion of nicotinamide stores, a finding which suggests synthesis of the vitamin by the intestinal bacteria.(4) An attempt to inhibit bacterial synthesis of nicotinamide in the intestinal tract was not reflected by a drop in the urinary excretion of N-methylnicotinamide (F2). (5) This suggests that nicotinamide synthesis in the intestinal tract was accomplished by bacteria which were not sensitive to sulfasuxidine in the doses used.