The effect of angiotensin on cerebral blood vessel was studied in unanesthetized fourty-one rabbits comparing with the effect of noradrenaline on it. Both internal carotid blood flow and external carotid blood flow were measured with two photoelectric drop-recorders simultaneously. Angiotensin of 0.1 μg injected into the Internal carotid artery caused marked decrease in the blood flow through this artery and then the same amount of angiotensin injected into the external carotid artery also caused the similar degree of decrease in the blood flow through the external carotid artery. On the other hand, noradrenaline of 5 μg injected into the internal carotid artery caused a minimal decrease or an actual increase in the blood flow through this artery, whereas the same amount of noradrena-line injected into the external carotid artery caused a marked decrease in the external carotid blood flow. Consequently angiotensin is considered to have a specific vesoconstrictive effect on the cerebral blood vessel, which is different from noradrenaline.