SummaryNormal subjects tolerate total obliteration of internal carotid artery, owing to three collateral channels : external carotid artery, Willis' polygone and peripheral arteries.A 50 % drop in systemic blood pressure makes the collateral system ineffective.All cerebral arteries, and particularly those constituting the collateral system, adapt their diameter to their blood flow. When the local blood flow decreases, as a result of atheromatous lesions located up-stream or of removal of an arterio-venous angioma situated downstream, the arterial diameter decreases and the vessel becomes less tortuous. These changes take place within about three weeks.When the cerebral blood flow decreases, the functional capacities of the cerebral cortex are altered. This state is reversible as long as the blood flow is maintained above a critical level.With arterial seriography, it is possible to measure the cerebral circulation time, which is found constant in the normal individual.In cerebral trauma, tumours and atheroma, blood can circulate at reduced flow in the whole brain or in some part of it. The circulatory defect parallels the evolution of coma, focal signs and electro-encephalographic abnormalities.Total obliteration of an artery by thrombosis is generally better tolerated than progressive stenosis by atheroma. In the latter case, the development of collateral channels is not as extensive.The syndromes of thrombosis and stenosis are differentiated.