The impact of non-methane hydrocarbons of natural and anthropogenic origin on the photochemistry of the nonurban lower atmosphere has been studied using a photochemical model with detailed planetary boundary layer transport. The research was undertaken to evaluate the increase in peroxy radical concentrations due to hydrocarbon oxidation and their contribution to observed imbalances in the NOx, O3 photostationary state (Parrish et al., 1986a). The results show that these hydrocarbons, at concentrations observed in the atmosphere, reduce the hydroxyl radical concentration in clean air and increase the peroxy radical concentration to levels that are needed to explain the observed imbalance in the photostationary state. For the conditions simulated here, natural hydrocarbons are the principal contributors to the increase in the peroxy radical concentration at low NOx mixing ratios. At higher NOx levels the anthropogenic hydrocarbons that scale with NOx gain in importance. Model simulations that include the hydrocarbons reproduce the observed dependence of the concentration of ozone on various NOx concentrations. ¿ American Geophysical Union 1987