A global three-dimensional photochemical tracer/transport model (CTM) of the troposphere has been developed. The model is based on the NASA/Goddard Institute for Space Studies (GISS) CTM with the incorporation of an extensive photochemical scheme. The model resolution is 8¿ latitude and 10¿ longitude with nine vertical layers below 10 hPa. One year of meteorological data from a free running GCM (NASA/GISS), including advective winds and convection frequency with 8-hour time resolution, is used as input. Transport of species by advection, convection, and diffusion is included in the model. The chemical scheme consists of 49 components, 85 thermal reactions, and 16 photolytic reactions. The chemical scheme is solved by the quasi steady state approximation (QSSA) method with iterations and chemical families, with a time step of 30 min. The model simulates well the lower tropospheric distribution of key species like carbon monoxide, nonmethane hydrocarbons (NMHCs), and ozone. The model is also able to simulate the important pattern of background NOx distribution Jaffe et al. this issue>. In the upper troposphere, coarse model resolution gives some discrepancies between modeled ozone concentrations and observations, especially at high latitudes. A global tropospheric ozone budget is presented. Net ozone production is found in the boundary layer and in the upper troposphere. In the middle free troposphere there is a close balance between chemical loss and production of ozone, giving a small net ozone loss. Hydroxyl (OH) concentrations are found to be sensitive to parameterization of cloud effects on photolysis rates. A global mean tropospheric OH concentration of 1.1¿106 molecules/cm3 is calculated, which is about 15% higher than recent estimates from analysis of CH3CCl3 observations indicate.¿ 1997 American Geophysical Union