1. The identification of a vascular wall renin angiotensin system and of angiotensin converting enzyme on the luminal surface of the endothelium in many tissues, including the brain, has stimulated research on the influence of the renin angiotensin system on regional blood flows. 2. In experimental studies inhibition of the angiotensin converting enzyme shifts the limits of cerebral blood flow autoregulation towards lower blood pressure values. 3. In patients with chronic arterial hypertension and in patients with chronic heart failure cerebral blood flow is not changed by acute or chronic angiotensin converting enzyme inhibition, despite in some cases pronounced reductions in the mean arterial blood pressure. Angiotensin converting enzyme inhibition does not change ischaemic regional cerebral blood flow in acute stroke. 4. It is concluded that following angiotensin converting inhibition cerebral blood flow is maintained at an unchanged level. The mechanism may include inhibition of locally produced angiotensin II leading to a selective dilation of larger cerebral arteries with a compensatory constriction of the smaller cerebral arteries.