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As human encroachment increases in the Amazon rain forest, it is important to determine how anthropogenic emissions of reactive gases affect regional atmospheric chemistry. In the present study, we investigate the extent to which urban air plumes modify the levels of ozone (O-3) and nitrogen oxides (NOx) in the downwind rain forest. The median mixing ratios of the background O-3, NOx, and NOx oxidation products (NOz) were 20 (15 parts per billion on a volume basis, ppbv), 0.6 ppbv (0.6 ppbv), and 1.0 ppbv (0.5 ppbv) during the dry (wet) season at the study site. Compared to the background environment, air plumes from the city of Manaus had enhanced median mixing ratios for O-3 and NOz by 30-50% and 40-90%, respectively. However, the enhancements of NOx in the air plumes were less than 20%, indicating that the majority of NOx was chemically converted to O-3 and NOx during transport. Results from a photochemical model showed that an injection of 8 ppbv of NOx into the rain forest can cause up to 260% and 150% increases in O-3 and hydroxyl radical (OH) levels compared to the background conditions, indicating the likely extent that NOx can modify the air quality and oxidative capacity in the Amazon rain forest. Slopes of the 0 3 -temperature linear relationships increased with NOx levels from 3.7 to 6.5 ppbv per degree Kelvin during the dry season and 1.7-5.5 ppbv per degree Kelvin during the wet season. Average rates of change of the slope with respect to NOz were approximately 1.8 and 2.3 times higher than those with respect to NOx for the dry and wet seasons. One key conclusion of this study is that NOz substantially contributed to the O-3 formation in response to temperature under enhanced NOx conditions in the forested environment.