Proximal segments of the cerebral arteries of aging rats ranging in age from 35 to 45 months were studied electron microscopically. Characteristic features of the medial smooth muscle cells showed single or multiple cytoplasmic focal degradations with electron density, and irregularly shaped and sized cytoplasmic small cave formations (foveations) similar to a spider's web. The latter was a characteristic feature in very old rats. Intercellular spaces were dilated with an increase in amount of basement membrane-like substances, either thick or multilaminated around the bizarrely atrophied smooth muscle cells. Bundles of filaments were present in the basal cytoplasm of the endothelial cells. Bifurcation pads ("Valve-like projections") at the branching points of the cerebral arteries also showed a smooth muscle cell necrosis with deposition of basement membrane-like substances and granulo-vesicular profiles around them, especially in the roots and margins of the bifurcation pads.