A structure has been found consistently in the wall of the internal carotid artery of the rat as it passes through the carotid canal. It consists of modified smooth muscle cells arranged longitudinally which exhibit abundant pinocytosis and are located close to endothelial cells, with which they make contact. These cells do not appear to be innervated. The closeness of this structure to the arterial lumen, into which it may actually protrude, suggests that it may be sensitive to vaso-active substances in arterial blood. If so, it is well place to affect cerebral blood flow.