A massive and transient increase in the formation of double-walled coated vesicles (DWCVs) from surface membranes during late cerebellar development is reported here. These structures are characterized by an outer vesicle (65 nm in diameter), bearing a 15 nm thick spiny coat, containing an inner vesicle (30 nm in diameter). DWCVs occur free in the cytoplasm or attached to the plasma membranes. In the latter case, the membrane of the outer vesicle can be seen to be an invagination of the plasmalemma of the parent cell process while the membrane of the inner vesicle is an evagination of the plasmalemma of the adjacent cell process. DWCVs were observed in a variety of cellular elements in the granular and molecular layers of immature mouse cerebellum, including axons, dendrites, glia and cell bodies. Morphometric analysis revealed that the number of DWCVs in cerebellar mossy terminals became elevated between 16 and 37 days of age and reached a peak 45--50 times higher at 20 days than at either 10 or 70 days of age. The data suggest that a massive conjugate internalization of apposed plasma membranes occurs during late postnatal development which may serve to remodel neural membranes.