From 10 minutes to 3(1/2) hours after the intraventricular injection into rats of 15 to 100 mg of ferritin, an appreciable fraction of the protein, visualized electron microscopically, traverses the ependymal epithelium by diffusing along the dense intercellular substance of the luminal open junction and thence, by circumventing discrete intercellular fusions which partition rather than seal the interspace. These partitions shunt additional protein into the cell, where ferritin is transported within pinocytotic vesicles to the lateral and basal plasma-lemma and, presumably, back into the interspace again. The basal interspace is irregularly distended by pools of moderately dense "filler" within which ferritin accumulates. The larger fraction of protein enters the ependyma by pinocytosis and is eventually segregated within membrane-enclosed organelles such as vacuoles, multivesicular bodies, and dense bodies, where the molecules may assume a crystalline packing. As a result of the accumulation of ferritin within these inclusions and within filler substance, only a small amount of protein remains to enter the underlying parenchyma. Presentation of ferritin to prefixed cells leads to a random dispersion of free cytoplasmic ferritin. This artifactual distribution in both prefixed and postfixed cells is concurrent with disruption of cell membranes.