The pharmacokinetics of nicotinamide were studied in four human volunteers after oral doses of 1-6 g. Plasma concentrations and clearance rates of the vitamin were found to be dose-dependent, with a half-life of approximately 7-9 h for the two highest doses administered (4 and 6 g), approximately 4 h with 2 g and approximately 1.5 h with a 1-g dose. Peak concentrations ranged from 0.7 to 1.1 mumol.ml-1 after a 6-g dose. The time to reach peak plasma concentration was dose independent with a broad range from 0.73 to 3 h. In this study, nicotinamide had no detectable effect on blood pressure, pulse or body temperature.