Measurements of ambient concentrations of reactive nitrogen oxides (NOy) were made in the Amazon rain forest, near Manaus, Brazil, contiguously at 39 m (above the canopy), and on several days and nights at 19 m (within the canopy). Concentrations were very low, typically 100-700 pptv, except for brief periods when up to 5000 pptv of NOy was observed, indicating polluted air from the urban area of Manaus. The forest was a net sink for NOy with the NOy flux=-7.6 (¿5.0)¿109 molecules cm-2 s-1 in unpolluted periods, even though soils emitted NO at a significant rate (8.9¿1.5¿109 molecules cm-2 s-1). The deposition rate for NOy appeared to be much larger during the daytime than at night, suggesting that uptake was controlled either by plant processes (stomatal opening) or by supply of reactive components of NOy (e.g., HNO3) during the daytime. Implications for regional and global atmospheric chemistry are discussed. ¿ American Geophysical Union 1990