All the rats were made hypertensive and fed beta-aminopropionitrile. Among them, cerebral aneurysms were never found in rats without carotid ligation. In rats with ligation of one or both common carotid arteries, cerebral aneurysms developed on the vessels where hemodynamic stresses were apparently increased. The relationship of hemodynamics in the circle of Willis to the development of cerebral aneurysms is discussed, being compared with findings in spontaneous lesions in man.