The presence in yeast and animal cells of diphosphopyridine nucleotide pyrophosphorylase suggested that the reaction catalyzed by this enzyme may represent the final step in the biogenesis of diphosphopyridine nucleotide (DPN). I An enzyme capable of catalyzing the synthesis of nicotinamide mononucleotide (NMN) from nicotinamide was sought and found in human erythrocytes (I). However, the fact, s that this enzyme exhibited a remarkably high K, for nicotinamide, that erythrocytes exhibit only very low DPN-pyrophosphorylase activity (1, 2) and that erythrocytes are capable of synthesis of DPN at low concentrations of nicotinic acid but not of nicotinamide (3, 4) suggested the existence of an alternate pathway for DPN synthesis. A preliminary report (5) described t’he appearance, in erythrocytes incubated with nicotinic acid-C14, of two presumed intermediates in DPN synthesis which were tentatively identified as nicotinic acid mononucleotidc and nicotinic acid-adcnine dinucleotide, the nicotinic acid analogues of NMN and DPN respectively. This report will present evidence for the structure of these intermediates and evidence for their participation in DPN synthesis in diverse biological systems.