A model for the photochemistry of the global troposphere constrained by observed cncentrations of H2O, O3, CO, CH4, NO, NO2, and HNO3 is presented. Data for NO and NO2 are insufficient to define the global distribution of these gaases but are nonetheless useful in limiting several of the more uncertain parameters of the model. Concentrations of OH, HO2, H2O2, NO, NO2, NO3, N2O5, HNO2, HO2NO2, CH3O2, CH3OOH, CH2O, and CH3CCl3 are calculated as functions of altitude, latidude, ans season. Results imply that the suorce for nitrogen oxides in the remote troposphere is geographically dispersed and surprisingly small, less than 107 tons N yr-1. Global surces for CO and CH4 are 1.5¿109 tons C yr-1 and 4.5¿108 tons C yr-1, respectively. Carbon monoxide is derived from combustion of fossil fuel (15%) and oxidation of atmospheric CH4(25%), withthe balance from burning of vegitation and oxidation of biospheric hydrocarbons. Production of CO in the northern hemisphere exceeds that in the southern hemisphere by about a factor of 2. Industrial and agricultural activities provide approximately half the global source of CO. Oxidation of CO and CH4 provides sources of tropospheric O3 similar in magnitude to loss by in situ chotochemistry. Observations of CH3CCl3 could offer an important check of the tropospheric model and results shown here suggest that computed concentrations of OH should be reliable within a factor of 2. A more definite test requires better definition of relaease rates for CH3CCl3 and improved measurements for its distribution in the atmosphere.