Cerebral aneurysms were induced in rats treated with unilateral ligation of the common carotid artery and hypertension produced by renal infarction with or without beta-aminopropionitrile, one of the lathyrogens. In this experiment, cerebral aneurysms developed more frequently than in the previous experiments in which hypertension induced by deoxycorticosterone and salt had been used. It was demonstrated that deoxycorticosterone was not essential to the development of the lesion. Regardless of its type, hypertension combined with carotid ligation played a primary role in the production of cerebral aneurysms in rats. Sodium chloride enhanced the systolic hypertension and thus increased the incidence of lesions. The role of hypertension in the development of cerebral aneurysms in humans is discussed.