The global distribution of NOx in the troposphere is calculated using a simple three-dimensional chemical tracer model. This model includes a simplified chemistry scheme for the tracers NOx≡NO+NO2 and HNO3, which are redistributed by advection, dry and wet convection, and large-scale diffusion. The sources of NOx considered are fossil fuel combustion, emissions from soil microbial activity, biomass burning, lightning discharges, emissions by aircraft, and downward transport from the stratosphere. Dry and wet deposition act as final sinks. At northern middle and high latitudes the calculated tropospheric NOx content is dominated by the surface sources, fossil fuel combustion in particular. In the tropical free troposphere, lightning discharges provide about 80% of the total NOx throughout the year. The zonally averaged fractional contribution of aircraft emissions strongly depends on the season. The largest contribution of this source, over 60%, occurs during January in the upper troposphere between 45 ¿N and 60 ¿N. The NO mixing ratios determined by the model show good overall agreement with vertical profiles measured during the Stratospheric Ozone Experiment (STRATOZ) III aircraft campaign. ¿ American Geophysical Union 1996