A saccular cerebral aneurysm was incidentally found at the bifurcation of a fenestrated anterior cerebral artery of 35-week-old Sprague-Dawley rat. The animal had been fed a normal laboratory diet. The internal lamina extended for a short distance into the neck of the aneurysm and then suddenly disappeared. The medial layer ended abruptly at the neck of the aneurysm. The aneurysmal wall consisted mainly of connective tissue, and of some poorly stained fragments of elastic lamina. No evidence of polyarteritis nodosa or any other inflammatory reaction was obtained. These findings were the same as those observed in man. Therefore, this aneurysm proved to be of truly spontaneous origin. In contrast to experimental aneurysms in rats and in monkeys, in which aneurysms are observed very often, degenerative changes of the elastic lamina in spontaneous aneurysm in the rat were confined to the neck of the aneurysm. This fact may explain partly the difference in frequency between spontaneous and experimental aneurysms in animals. The present study indicates that degenerative changes of the elastic lamina play a very important role in the development of saccular cerebral aneurysms.