Inhibition of angiotensin converting enzyme (ACE) shifts the limits of cerebral blood flow autoregulation towards lower blood pressure values. In patients with chronic arterial hypertension and in patients with chronic heart failure cerebral blood flow is not changed by acute or chronic ACE inhibition, despite, in some cases, pronounced reductions in the mean arterial blood pressure. ACE inhibition does not change ischaemic regional cerebral blood flow in acute stroke. It is concluded that following ACE inhibition cerebral blood flow is maintained at an unchanged level and especially well maintained in case of blood pressure drop.