Accumulating evidence suggests that active maintenance of optimal levels of the essential pyridine nucleotide, nicotinamide adenine dinucleotide (NAD+) is beneficial in conditions of either increased NAD+ turnover or inadequate synthesis, including Alzheimer’s disease and other neurodegenerative disorders and the aging process. While studies have documented the efficacy of some NAD+ precursors such as nicotinamide riboside (NR) in raising plasma NAD+, no data are currently available on the fate of directly infused NAD+ in a human cohort. This study, therefore, documented changes in plasma and urine levels of NAD+ and its metabolites during and after a 6 h 3 μmol/min NAD+ intravenous (IV) infusion. Surprisingly, no change in plasma (NAD+) or metabolites [nicotinamide, methylnicotinamide, adenosine phosphoribose ribose (ADPR) and nicotinamide mononucleotide (NMN)] were observed until after 2 h. Increased urinary excretion of methylnicotinamide and NAD+ were detected at 6 h, however, no significant rise in urinary nicotinamide was observed. This study revealed for the first time that: (i) at an infusion rate of 3 μmol/min NAD+ is rapidly and completely removed from the plasma for at least the first 2 h; (ii) the profile of metabolites is consistent with NAD+ glycohydrolase and NAD+ pyrophosphatase activity; and (iii) urinary excretion products arising from an NAD+ infusion include NAD+ itself and methyl nicotinamide (meNAM) but not NAM.