The role of nitrogen oxides in the tropospheric photochemical system is re-evaluated in the light of recent measurements of the rate constants for two key reactions. A model for nitrogen oxides is discussed which yields surface NOx (NO+NO2) levels approaching 1 ppb in NOx source regions but less than 0.1 ppb outside source regions. Applying the new rate coefficients implies increased radical concentrations and a more intense O3 CO photochemistry. Even for densities of 0.1 ppb or less, NOx still leads to significant local O3 production and conversion of HO2 to OH. Unrealistic O3 profiles are obtained with the new rate coefficients for surface NOx densities about 1 ppb, while reasonable agreement with observation is obtained with lower NOx densities. Feedback processes between CO, NOx, OH, and CH4 are also discussed.