The influence of engine variables on the concentration of oxides of nitrogen present in the exhaust of a multicylinder engine was studied. The concentrations of nitric oxide (NO) were measured with either a mass spectrometer or a non-dispersive infrared analyzer. The NO concentration was low for rich operation (deficient in oxygen) and increased with air-fuel ratio to a peak value at ratios slightly leaner than stoichiometric proportions. A further increase in air-fuel ratio resulted in reduced NO concentrations. Advanced spark timing, decreased manifold vacuum, increased coolant temperature and combustion chamber deposit buildup were also found to increase exhaust NO concentration. These results support either directly or indirectly the hypothesis that exhaust NO concentration is primarily a result of the peak combustion gas temperature and the available oxygen.The NO concentration of the exhaust from an individual cylinder is a function of the air-fuel ratio of the charge that the individual cylinder receives. Since the NO concentration as a function of air-fuel ratio is highly non-linear, it was concluded that the NO concentration of the conglomerate exhaust is a function of distribution as well as overall air-fuel ratio.The NO concentration of the gases expelled from an engine cylinder varies with time. The last portion to be expelled is, at least under some operating conditions, lower in NO concentration than the average of the well mixed exhaust gas from that cylinder. These results can be explained by flame quenching resulting from the relatively cold combustion chamber walls.