The ependymal lining of the aqueduct of the rabbit changes considerably in the perinatal period from a thick stratified layer of relatively undifferentiated polygonal cells to cells of the adult type. Elaborate cell membrane infoldings and cilia form on the surface cells just before birth. After birth, the lining has diminished to a pseudostratified ciliated columnar layer of cells, among which three ependymal variants can be distinguished. The first is a simple columnar cell which exists in somewhat modified form in the mature rabbit. The other types are most common in the neonatal period. They are ependymal cells exhibiting long peripherally directed processes which appear to correspond with Horstmann's light microscope descriptions of the ependymal tanycyte and the ependymal astrocyte in lower vertebrates. The process of the ependymal tanycyte can be differentiated from other processes in the neuropil by its characteristically dense appearance. Dilated cisternae of endoplasmic reticulum, numerous longitudinally arranged mitochondria and fine filaments are present in the base of the cell and extend into the proximal portion of the process. The number of organelles diminishes distally where the process terminates by splitting into multiple end feet near or on capillary walls. The process of the ependymal astrocyte differs in that it is branched and is paler due to the presence of fewer organelles. Agranular vesicles and short randomly-disposed filaments are embedded in a finely granular ground substance which characteristically has a “ground glass” appearance. The lateral cell walls of the perinuclear area and also of the main process are highly elaborated, forming thin regular cytoplasmic plates or finger-like projections. It is suggested that these elaborate plications of the cell membrane may represent a stage in the formation and storage of its extensive smaller branches radiating out from the main process.