Saccular cerebral aneurysms were successfully induced in two monkeys treated with ligation of the common carotid artery, experimental hypertension, and beta-aminopropionitrile feeding. The cerebral aneurysms developed on the large arteries at the base of the brain, such as the anterior communicating artery and the internal carotid artery at the origin of the posterior communicating artery. Because of the similarity of the monkey to man as a species, the present results strongly suggest the significance of postnatal aggravating factors in the development of cerebral aneurysms in man.