To estimate the effect of subsonic and supersonic aircraft exhaust on the stratospheric concentration of NOy, we employ a trajectory model initialized with air parcels based on the standard release scenarios. The supersonic exhaust simulations are in good agreement with two-dimensional (2-D) and 3-D model results and show a perturbation of about 1--2 ppbv of NOy in the stratosphere. The subsonic simulations show that subsonic emissions are almost entirely trapped below the 380 K potential temperature surface. Our subsonic results contradict results from most other models, which show exhaust products penetrating above 380 K, as summarized by Danilin et al. <1998> and Intergovernmental Panel on Climate Change (IPCC) <1999>. The disagreement can likely be attributed to the excessive model diffusion of the strong vertical gradient in NOy that forms at the boundary between the emission zone and the stratosphere above 380 K. Our results suggest that previous assessments of the impact of subsonic exhaust emission on the stratospheric region above 380 K should be considered to be an upper bound. ¿ 2000 American Geophysical Union