This paper presents experimental results of studies investigating the effect of nitric oxide (NO) on the autoignition chemistry of a primary reference fuel blend with an octane rating of 87 in a motored engine. The experiments were conducted over a range of operating conditions in a single cylinder research engine at compression ratios of 5.2 and 8.2. The inlet manifold was heated and supercharged to pre-stress the fuel-air mixture in order to produce in-cylinder pressure and temperature histories similar to practical engines. The exhaust gas carbon monoxide concentration was monitored and used as a measure of overall reactivity. In-cylinder pressure histories were also recorded and processed to calculate in-cylinder temperature histories. Results showed that at low manifold temperatures, below that necessary to produce negative temperature coefficient behavior, up to 100 ppm of NO promoted reactivity, whereas higher concentrations retarded the reactivity. In the negative temperature coefficient region, it was found that all NO concentrations up to 300 ppm enhanced reactivity, and at the 8.2:1 compression ratio, 400 ppm of NO produced autoignition. The possible causes of this behavior are discussed in terms of the low and intermediate temperature hydrocarbon oxidation mechanism.