A global three-dimensional model of tropospheric chemistry driven by reanalyzed European Centre for Medium-Range Weather Forecasts meteorological data is used to examine the sources of O3, CO, and nitrogen oxides (NOx = NO + NO2) in the South Pacific troposphere during the NASA Pacific Exploratory Mission to the Tropics (PEM-Tropics A) in September--October 1996. Aircraft observations up to 12 km during that mission revealed considerable biomass burning influence on O3 and CO in terms of elevated pollution layers and regional enhancements. The model reproduces the long-range transport of biomass burning effluents from southern Africa and South America in the westerly subtropical flow over the South Pacific. Meteorological conditions in 1996 were particularly favorable for this transport. Africa and South America make comparable contributions to the biomass burning pollution over the South Pacific; the contribution from Australia and Indonesia is much less. Biomass burning dominates the supply of NOx in the lower troposphere over the South Pacific (through long-range transport and decomposition of peroxyacetylnitrate), but lightning dominates in the upper troposphere. Observations in PEM-Tropics A and elsewhere indicate low HNO3/NOx concentration ratios and an imbalance in the chemical budget of NOx in the upper troposphere. We reproduce these observations in our model and show that they reflect the subsidence of primary NOx injected by lightning into the uppermost troposphere, rather than any fast chemistry recycling HNO3 to NOx. We find that biomass burning and lightning made similar contributions to O3 production over the South Pacific during PEM-Tropics A. Biomass burning plumes sampled in PEM-Tropics A contained little NOx, and the O3 enhancements observed in these plumes originated from production over the source continents rather than over the South Pacific.