Stratospheric NOy fields calculated using a zonally averaged interactive chemistry-radiation-dynamics model show significant sensitivity to the model CO2. Modeled upper stratospheric NOy decreases by about 15% in response to CO2 doubling, mainly due to the temperature decrease calculated to result from increased CO2 cooling. The abundance of atomic nitrogen, N, increases because the rate of the strongly temperature dependent reaction N+O2→NO+O decreases at lower temperatures. Increased N leads to an increase in the loss of NOy which is controlled by the reaction N+NO→N2+O. The decrease in NOy due to the lowered temperatures is partially compensated by changes in the residual circulation. In addition, the NOy reduction is shown to be sensitive to the NO photolysis rate. ¿ 1998 American Geophysical Union