Measurements indicate that ambient NOx(NO + NO2) concentrations vary over at least an order of magnitude in the troposphere. Thus it is of interest to study atmospheric chemistry with NOx concentrations covering this range. In this paper we present steady state calculations which show that as NOx concentrations are increased from very low values, the hydroxyl radical concentration [OH] increases at first and then decreases. This change of behavior occurs at [NO2] ∼ 0.23 ppb in our model. Also, the response of the atmosphere to increased CO fluxes undergoes a qualitative change as a function of [NOx]. At low levels of [NOx], an increase in CO fluxes into the atmosphere depletes [OH] and increases [CH4] and [H2], as reported in the literature. However, at high values of [NOx], increased CO fluxes give rise to an increase in [OH] and, consequently, a decrease in [CH4] and [H2].