A theoretical estimate of the quantity of nitrogen oxides NO and NO2 produced by the corona phase of a lightning flash has been carried out. The procedure makes use of physical simulations of positive leader and streamer propagation, coupled to a chemical kinetics model including 35 species such as ions, radicals, and neutral molecules. The effects of molecular and turbulent diffusion in thunderstorms are also modeled. A parametric study with various values of pressure and temperature leads to the conclusion that the effect of the corona phase on NOx production is negligible compared to that of the return stroke of the flash. ¿ 1998 American Geophysical Union