A three-dimensional perspective on the stratosphere-to-troposphere exchange is provided by trajectory calculations based on winds from the GFDL SKYHI general circulation model. For particles initialized at the 350 K isentropic surface, the exchange is initially (for up to 30 days) more vigorous in the summer hemisphere, in agreement with the isentropic results of Chen [1995>. However, on longer time scales the relative importance of the exchange in the winter hemisphere increases, as particles in that hemisphere descend more rapidly towards the extratropical tropopause. These results illustrate the importance of cross-isentropic transport for the stratosphere-to-troposphere exchange on seasonal time scales. ¿ American Geophysical Union 1996