The atmospheric composition over the Amazon forest during the wet season is simulated with a one-dimensional photochemical model for the planetary boundary layer (PBL) extending from the ground to 2000-m altitude. The model is constrained and evaluated using observations from the ABLE 2B field expedition. Results indicate that only ≈20% of NOx emitted by soils is exported to the atmosphere above the forest canopy. The balance is deposited to vegetation before leaving the canopy layer. The small NOx flux that escapes from the canopy is nevertheless sufficient to account for the low NO concentrations observed in the PBL. Decomposition of peroxyacetylnitrate (PAN) supplied from aloft provides only a minor source of NOx in the PBL, although it could provide the major source of NOx at higher altitudes. Soil emission can account for only a portion of NOy observed over the forest. Organic nitrates of nonbiogenic origin likely account for the balance of NOy. Enhancements of CO observed in the PBL cannot be explained by oxidation of biogenic hydrocarbons, and appear to reflect direct emission of CO by the forest ecosystem. Concentrations of O3 in the PBL are regulated largely by transport from aloft and deposition to the canopy, with little net influence from photochemistry. Ozone is photochemically produced immediately above the forest where NO concentrations are relatively high, but is photochemically consumed in the upper portion of the PBL. ¿ American Geophysical Union 1990