A photochemical box model with CO-CH4-NOy-H2O chemistry is used to calculate the diurnally averaged net photochemical rate of change of ozone (hereinafter called the chemical ozone tendency) in the troposphere for different values of parameters: NOx and ozone concentration, temperature, humidity, CO concentration, and surface albedo. To understand the dependency of the chemical ozone tendency on the input parameters, a detailed sensitivity study is performed. Subsequently, the expected variations of the ozone tendencies with altitude, latitude, and season are analyzed. The magnitude of the tendency decreases rapidly with height mostly as a result of lower absolute humidity and temperature. In the upper troposphere (at 190 mbar) the maximum tendencies are below 2 parts per billion by volume/day. Lower temperature and specific humidity cause a shift of the value of NOx at which the ozone production balances the destruction of ozone (balance point) to lower NOx values; these two parameters are also, to a large extent, responsible for lower magnitudes of the tendency at higher latitudes and in winter. In the upper troposphere we find that the net tendency is at least as sensitive to variations in H2O concentration as to NOx. This suggests a possible synergism between direct NOx pollution by aircraft and the indirect modification of H2O by climate change. In the second part of the paper the box model calculated rates are used as ozone's chemical tendency terms during a simulation conducted with the three-dimensional global chemistry transport model (GCTM). The box model is used to calculate the tendencies as a function of NOx and ozone at all tropospheric levels of the GCTM, at nine latitudes and for four seasons using zonally and monthly averaged data: water vapor and temperature from observations and model CO. These tables together with the NOx fields obtained in an earlier GCTM simulation are used in the GCTM simulation of O3 if nonmethane hydrocarbon levels are low. The global monthly averaged chemical ozone tendency fields saved during the simulation are presented and analyzed for the present-day and preindustrial conditions. The chemical tendency fields show a strong correlation with the NOx fields. In contrast with the lower and middle troposphere where the tendencies are negative in remote regions over the oceans, in the upper troposphere, where NOx is generally greater than 50 parts per trillion by volume and the balance point is low, the tendencies are generally small but positive. The GCTM simulations of the preindustrial ozone show that in the upper troposphere the present-day ozone tendencies are greater than the simulated preindustrial tendencies. In the boundary layer and in the midtroposphere the present-day tendencies are greater near anthropogenic NOx sources and smaller (generally more negative), due to higher ozone levels, in regions not affected by these sources.¿ 1997 American Geophysical Union