A three-dimensional off-line simulation of the transport of the long-lived source gas N2O in the summer stratosphere has been made with a global model. We found long-lived westward-propagating patterns resulting from the slow advection of coarsely-mixed structures of ex-vortex air and midlatitude air. The mixing of the air masses with these two different characteristics is a slow process that lasts from spring until late summer. In the regime of summer easterlies above 20 km, we found evidence of long-lived, fossil debris from the polar vortex even in August. ¿ 2001 American Geophysical Union