Valves associated with major arterial branches at the base of the brain in rats were studied after fixation by perfusion with buffered aldehydes. These endothelial covered structures are entirely intimal. All cellular and non-cellular elements of their core are laminated, including the PAS positive ground substance. Smooth muscle and nerve endings are not demonstrable. Reticular fibers form a substantial sheath around the base of each valve. Their location and morphology suggest that they may serve a regulatory function in maintaining proper intracerebral blood flow.