The development and validation of simulation model for estimating ground level concentrations of photochemical pollutants is described. This model is based on the finite difference solution of the equations of conservation of mass, using the method of fractional steps. The bulk of the effort is developmental, involving the compilation of a comprehensive source emissions inventory, the development and validation of a kinetic mechanism for photochemical reactions, the adaptation of the method of fractional steps for use in the solution of the governing equations, and the preparation of maps displaying spatial and temporal variations in wind speed and direction and in the height of the inversion base. Although a validated kinetic mechanism has been developed and incorporated in the simulation model, validation efforts have thus far been restricted to carbon monoxide. Provisional validation results for the Los Angeles Basin are presented.