The concentrations of oxides of nitrogen (NOx = NO + NO2) in engine crankcases interest lubricant technologists because NOx reactions with fuel and lubricant components are partially responsible for engine deposits and oil deterioration. To provide information hitherto unavailable in the literature, concentrations of NOx and NO2 were measured in the crankcase gas of a multi-cylinder engine operated at a variety of steady-state conditions.The concentrations of NOx and NO2 measured in the crankcase gas followed the same trends with changes in engine operating variables as the concentrations of NOx and NO2 in the exhaust gas. Specifically, increasing intake manifold pressure, spark advance, and compression ratio, generally increased the concentrations of NOx and NO2. Increasing the percent exhaust gas recirculation, however, decreased NOx and NO2 concentrations. Changing air-fuel ratio (A/F) from rich to lean first increased and then decreased crankcase NOx and NO2 concentrations; the maximum NOx and NO2 concentration occurred at an A/F of about 16. Engine speed affected NOx and NO2 concentrations slightly and inconsistently.Measured NOx and NO2 concentrations in the crankcase gases were in the range 8.7 to 376 and 0.3 to 125 ppm, respectively. Crankcase NOx and NO2 concentrations could not be predicted from those measured in the exhaust gases because the sources of NOx and NO2 are not identical for both crankcase and exhaust gases. Quench gases in the combustion chambers appear to be a significant source of crankcase NOx, and probably a primary source of crankcase NO2. Some NO2 is also formed in the crankcase itself from oxidation of NO.