The sum of the urinary excretion of nicotinamide and its catabolites, which are metabolites of NAD and NADP, were observed to have clear diurnal variations in human urine. Then, we examined whether NAD and NADP in blood also showed the diurnal variation. All subjects were housed in the same facility and given the same diet during the experiment. In addition, we examined whether diurnal variations were affected by the intakes of dietary nicotinamide or not. As a result, neither the NAD nor the NADP content of the blood shows the diurnal variation regardless of the administered amount of nicotinamide. The concentrations of NAD and NADP did not increase according to the intake of nicotinamide. The existence of a mechanism by which NAD and the NADP levels of the blood are constantly maintained by the adjustment of the amount of excretion to the urinary bladder, was suggested.