The normal configuration of the arterial pattern at the base of the brain in rhesus monkeys, dogs, sheep, goats and rabbits is described. The length and diameter of all vessels of the circulus arteriosus cerebri are measured. Variations are compared in relation to that of the typical pattern present in humans. In rhesus monkeys, dogs and rabbits, the circle of Willis is almost similar to that in human beings except that in these animals, (i) the anterior cerebral arteries join each other to form a single median vessel; (ii) in dogs, rabbits, goats and sheep the anterior cerebellar artery is a branch of posterior cerebral artery, and (iii) in goats and sheep, a considerable length of the internal carotid artery contributes to the formation of circulus arteriosus while the posterior cerebral artery arises from the posterior communicating artery.