Nitrogen oxides (NO_X) emitted fromhuman activities are believed to regulate the atmospheric oxidation capacity of the troposphere. However, observational evidence is limited for the low-to-median NO_X concentrations prevalent outside of polluted regions. Directly measuring oxidation capacity, represented primarily by hydroxyl radicals (OH), is challenging, and the span in NO_X concentrations at a single observation site is often not wide. Concentrations of isoprene and its photo-oxidation productswere used to infer the equivalent noontime OHconcentrations. The fetch at an observation site in central Amazonia experienced varied contributions from background regional air, urban pollution, and biomass burning. The afternoon concentrations of reactive nitrogen oxides (NO_y), indicative of NO_X exposure during the preceding few hours, spanned from 0.3 to 3.5 parts per billion. Accompanying the increase of NO_y concentration, the inferred equivalent noontimeOHconcentrations increased by at least 250%from0.6 × 10⁶ to 1.6 × 10⁶ cm^-3. The conclusion is that, compared to background conditions of low NO_X concentrations over the Amazon forest, pollution increased NO_X concentrations and amplified OH concentrations, indicating the susceptibility of the atmospheric oxidation capacity over the forest to anthropogenic influence and reinforcing the important role of NO_X in sustaining OH concentrations.