Abstract. The intracellular sorting of EGF-receptor complexes (EGFoRC) has been studied in human epidermoid carcinoma A431 cells. Recycling of EGF was found to occur rapidly after internalization at 37"C. The initial rate of EGF recycling was reduced at 18°C. A significant pool of internalized EGF was incapable of recycling at 18°C but began to recycle when cells were warmed to 37°C. The relative rate of EGF outflow at 370C from ceils exposed to an 18°C temperature block was slower (tla = 20 min) than the rate from cells not exposed to a temperature block (tv2 = 5-7 min). These data suggest that there might be both short-and long-time cycles of EGF recycling in A431 cells. Examination of the intraceUular EGF-RC dissociation and dynamics of short-and long-time recycling indicated that EGF recycled as EGF-RC. Moreover, EGF receptors that were covalently labeled with a photoactivatable derivative of 125I-EGF recycled via the long-time pathway at a rate similar to that of t25I-EGF. Since EGF-RC degradation was also blocked at 18°C, we propose that sorting to the lysosomal and long-time recycling pathway may occur after a highly temperature-sensitive step, presumably in the late endosomes.