Aircraft measurements of peroxyacetyl nitrate (PAN) were performed for the first time over the Brazilian Amazon Basin during the wet season (April–May 1987) as part of the NASA Atmospheric Boundary Layer Experiment (ABLE 2B) expedition. In addition to tropical measurements, free tropospheric latitudinal profiles were also obtained during transit flights to and from Manaus, Brazil. Complementing PAN were measurements of NO, O3, CO, C2Cl4, radon, and a variety of other chemical and meteorological parameters. Over the Amazon Basin, PAN was present at a mixing ratio of 5 to 125 ppt. Despite strong local and regional convective activity, a distinct vertical structure with highest mixing ratios aloft was observed. Median PAN mixing ratios of 12, 20, and 48 ppt were present in the 0- to 2-, 2- to 4-, and 4- to 6-km height intervals, respectively. Data collected during the cross-basin flights showed that the PAN mixing ratio was highest over the rain forest and declined eastward toward the Atlantic Ocean. Over the Atlantic, PAN was low and appeared to be uniformly distributed with height. Above the Amazon forest, PAN was as much as 5 times more abundant than NOx with the largest PAN/NOx ratios occurring at the highest altitudes. Both PAN and possibly the PAN/NOx ratio showed a latitudinal dependence, with decreasing values from the northern midlatitudes to the tropics. Free tropospheric (4–6 km) PAN mixing ratios were found to be strongly correlated with those of O3. Preliminary modeling results indicate that a sizeable fraction of NOx and HNO3 in the free troposphere could result from PAN decomposition alone. The primary source of free tropospheric PAN observed over the Amazon Basin is not well understood. Large-scale transport from the upper tropospheric PAN reservoir present at the higher northern latitudes, and precursor emissions of nonmethane hydrocarbons from the forest and NOx from soil and lightning clearly play an important role.