Walls of the third and fourth ventricle of the rabbit, cat, and squirrel monkey brain have been studied with the scanning electron microscope. The major parts of the ventricular walls are covered by a dense layer of cilia. The surface of the choroid plexus and of distinct areas known as the circumventricular organs is free of cilia. Ventricular surfaces of the following circumventricular organs were examined: organum vasculosum of the lamina terminalis, subfornical organ, subcommissural organ, and area postrema. In choroid plexus, organum vasculosum, and area postrema there are tight junctions between the nonciliated ependymal cells. This is in contrast to areas covered by ciliated ependyma where there are open interependymal gap junctions. The scanning electron microscopic observation of regional structural specialization in the ventricular surface thus seems to support the concept of regional functional specialization.