In the free troposphere, injection from the stratosphere, emissions at the surface transported upward by dynamic processes, aircraft emissions and production in lightning strokes are the main sources of oxides of nitrogen (NOx). The global source of NOx production by lightning is not well known and estimates vary from 3 to 650 Mt NO2/yr. In this paper the role of regional and episodic emissions of NOx from lightning is examined with a three-dimensional chemistry transport model, and a parameterization of the emissions is proposed, linking it to the intensity of latent heat generation in convection calculated in a numerical weather prediction model. The parameterization is scaled to give an annual global emission of 16 Mt NO2/yr. The resulting distribution of the concentration of NOx, ozone, OH and nonmethane hydrocarbons is compared for the free troposphere over the northern hemisphere with the results of a calculation where the total NOx emissions from lightning were about the same but were fixed in time and space over the 18 days' calculation (June 18 to July 5, 1995). The maximum differences in the free tropospheric concentrations are significant: for NOx, OH and nonmethane hydrocarbon of the same order of magnitude as the concentrations themselves, for ozone 1 order of magnitude less than the ozone concentration. This means that NOx emissions from lightning may cause a variability in free tropospheric composition which makes it quite difficult to distinguish the importance of other sources of free tropospheric NOx from that of lightning in periods when lightning occurs.¿ 1997 American Geophysical Union