Large-scale density structures are a common feature in the high-latitude ionosphere. They have been observed in the dayside cusp, polar cap, and nocturnal auroral region. Relative to background densities, the perturbations associated with large-scale structures vary from about 10% to a factor of 100. The lifetime and transport characteristics of ''large'' ionospheric structures (factor of 10 to 100) were studied with the aid of a three-dimensional time-dependent ionospheric model. Both density depletions and enhancements were considered. A density structure was created at a specific location in the high latitude F region and the subsequent evolution was followed for different seasonal and solar cycle conditions as well as for different orientations of the interplanetary magnetic field (IMF), i.e., different convection patterns. Depending on the IMF, horizontal plasma convection can cause an initial structure to break up into multiple structures of various sizes, remain as a single distorted structure, or become stretched into elongated segments. The lifetime of an F region density structure depends on several factors, Including the initial location where it was formed, the magnitude of the structure, season, solar cycle, and covection pattern (IMF). For example, in summer the effects of a large density structure can disappear in a few hours or last as long as 9 hours, while in winter the effects can persist for 24 hours. The passage of perturbed plasma flux tubes through sunlit and auroral regions can significantly increase the lifetime of plasma enhancements and can significantly reduce the lifetime of plasma depletions. ¿ American Geophysical Union 1987