The cerebrovascular effect of converting enzyme inhibition was investigated in normal volunteers, in hypertensive patients and in patients with severe chronic heart failure. Cerebral blood flow (CBF) was measured by single photon emission tomography of the uptake and wash-out of inhaled xenon-133. In some of the patients, CBF was also monitored by repeated determinations of the cerebral arterio-venous oxygen difference. In the normal volunteers and the hypertensive patients captopril (50 mg) caused a downward shift of the lower limit of CBF autoregulation of 10-15 mmHg. In some of the patients with chronic heart failure, captopril (6.25 mg) induced a marked decrease of the arterial blood pressure. Regional and average CBF, however, remained constant. These findings indicate that captopril treatment may protect the brain against ischaemia during sudden decreases of the blood pressure by a shift of the lower limit of CBF autoregulation.