A simple quasi-two-dimensional model is used to study the zonal distribution of NO _x . The model includes vertical transport in the form of eddy diffusion and deep convection, zonal transport by a vertically uniform wind, and a simplified chemistry of NO, NO₂, and HNO₃. The NO _x sources considered are surface emissions (mostly from the combustion of fossil fuel), lightning, aircraft emissions, and downward transport from the stratosphere. The model is applied to the latitude band of 40° to 50°N during the month of June; the contributions to the zonal NO _x distribution from the individual sources and transport processes are investigated. The model predicted NO _x concentration in the upper troposphere is dominated by air lofted from the polluted planetary boundary layer over the large industrial areas of eastern North America and Europe. Aircraft emissions are also important and contribute on average 30%. Stratospheric input is minor about 10%, less even than that by lightning. The model provides a clear indication of intercontinental transport Of NO _x and HNO₃ in the upper troposphere. Comparison of the modeled NO profiles over the western Atlantic with those measured during STRATOZ III in 1984 shows good agreement at all altitudes.