During the two and a half years which have elapsed since Elvehjem and his associates1 demonstrated the nutritional significance of nicotinic acid, much progress has been made in the clinical application of their discovery and much better insight into certain deficiency states has been gained. It was at once suspected that the importance of nicotinic acid in nutrition was related to its presence as the reactive fraction of the coenzymes diphosphopyridine nucleotide and triphosphopyridine nucleotide. Both coenzymes function in a number of dehydrogenating or oxidizing reactions, and the diphosphopyridine nucleotide may also act as a phosphate carrier. Their activity in the intermediate metabolism of dextrose is probably of the greatest clinical interest. The chemical reactions may be tentatively summarized. After the phosphorylation of hexose to hexose diphosphate and subsequent cleavage to triose phosphate, the pyridine nucleotides oxidize triose phosphate, losing oxygen, which is replaced by the oxidation of flavoprotein.