Saccular cerebral aneurysms are induced in rats treated with ligation of one or both of the common carotid arteries, experimental hypertension, and beta-aminopropionitrile feeding. Combination of ligation of the carotid artery and experimental hypertension is the minimal requirement for inducing aneurysms within a few months. beta-aminopropionitrile makes the arterial wall fragile, increasing the incidence of aneurysmal development. Induced aneurysms are strongly related to haemodynamic stresses. They are located on the large arteries at the base of the brain. Some of them apparently originate from the apex of bifurcation. The macroscopic and microscopic findings are generally in accordance with those of spontaneous lesions in man. The results of electron microscope and histochemical studies indicate the participation of leukocytes and their lysosomal enzymes in the development and growth of aneurysms. Adventitial cells are also suggested to be responsible for the growth of aneurysms.