Our first intercomparison/assessment of the effects of a proposed high-speed civil transport (HSCT) fleet on the stratosphere is presented. These model calculations should be considered more as sensitivity studies, primarily designed to serve the following purposes: (1) to allow for intercomparison of model predictions; (2) to focus on the range of fleet operations and engine specifications giving minimal environmental impact; and (3) to provide the basis for future assessment studies. The basic scenarios were chosen to be as realistic as possible, using the information available on anticipated developments in technology. They are not to be interpreted as a commitment or goal for environmental acceptability. Ko, Malcolm K. W. and Weisenstein, Debra and Jackman, Charles H. and Douglass, Anne R. and Bureske, K. and Weubbles, Donald J. and Kinnison, Douglas E. and Brasseur, G. and Pyle, J. and Jones, Anna Goddard Space Flight Center; Langley Research Center ...