An hydraulically actuated valve was used to sample directly the combustion chamber of a spark ignition engine in order to determine the time history of gas composition. The cold wall was found to influence equally carbon monoxide, carbon dioxide, and nitric oxide formation, as it has been previously shown to influence unburned hydrocarbons. Results indicated that sample fraction should be at least l% of cylinder content if the chemical analysis is to reflect conditions away from the wall reliably. There was evidence that large gradients in composition are created across the combustion chamber and these gradients persist throughout the expansion. Examination of engine operating variables, such as spark advance, compression ratio, mixture strength, and speed, provided more evidence of the role which chemical kinetics plays in determining the concentration of nitrogen oxides appearing in engine exhaust.