Experimental cerebral aneurysms in monkeys induced by treating them with unilateral carotid ligation, renal hypertension, and beta-aminopropionitrile feeding were studied macroscopically and with light microscopy. Macroscopically, 13 cerebral aneurysms were observed in six of seven monkeys. Microscopically, the elastic lamina and the media ended around the beginning of the aneurysms. The thin aneurysmal walls consisted of fibrous connective tissues. All findings were generally in accordance with spontaneous lesions in humans. The induced cerebral aneurysms in primates are thought to be a suitable experimental model to clarify the pathogenesis of the disease.