A spectrophotometric method is presented for the determination of N′-methyl-2-pyridone-5-carboxamide and N′-methyl-4-pyridone-3-carboxamide together in human plasma. Recovery was 86% for the former compound, 94% for the latter. The coefficient of variation was 5.6%, and the sensitivity of the method 43 μg%. In order to provide a quantitative assessment of the effect of interfering substances, the terms interference coefficient ( I), maximum permissible interference ( I max ) and maximum permissible contamination ( MPC) were introduced. The use of these terms in analyzing the interference of other nicotinamide metabolites, and of theophylline and caffeine, is illustrated. Using this method pyridones were not detected in significant amounts in the fasting state. Following the oral administration of 400 mg nicotinamide to 12 normal subjects the mean pyridone concentration rose to a maximum of 510 μg% at 4 h, and was still elevated at 8 h.