The photochemistry of the troposphere over the South Atlantic Basin is examined by modeling of aircraft observations up to 12-km altitude from the TRACE-A expedition in September-October 1992. A close balance is found in the 0-12 km column between photochemical production and loss of O3, with net production at high altitudes compensating for weak net loss at low altitudes. Simulation of H2O2, CH3OOH, and CH2O concentrations measured aboard the aircraft lends confidence in the computations of O3 production and loss rates. Influx from the stratosphere is negligible as a source of O3 or NOx to the 0-12 km column. The primary sources of NOx appear to be continental (combustion, lightning, soils) and include a major contribution from biomass burning. There is evidence that NOx throughout the 0-12 km column is recycled from its oxidation products rather than directly transported from its primary sources. There is also evidence for rapid conversion of HNO3 to NOx in the upper troposphere by a mechanism not included in current models. A general representation of the O3 budget in the tropical troposphere is proposed that couples the large-scale Walker circulation and in situ photochemistry. Deep convection in the rising branches of the Walker circulation injects NOx from combustion, soils, and lightning to the upper troposphere, leading to O3 production; eventually the air subsides and net O3 loss takes place in the lower troposphere, closing the O3 cycle. This scheme implies a great sensitivity of the oxidizing power of the atmosphere to NOx emissions in the tropics.