Cerebral blood flow (CBF) was investigated in 8 patients with chronic heart failure (CHF) (functional class III) and in twelve controls before and after administration of 6.25 mg and 25 mg captopril, respectively. In four controls, CBF was measured by the intracarotid xenon-133 (133Xe) injection technique using stationary external detectors, while inhalation of 133Xe and single photon emission computer tomography was used in the remaining cases. In the control group, the cerebral metabolic rate for oxygen was calculated from measurements of the arterio-venous oxygen difference as well. Mean CBF was significantly (P less than 0.01) lower in the patients with CHF as compared to our controls. Following captopril administration the mean arterial blood pressure decreased in the CHF patients, ranging from 5 to 40%. Three patients showed decreases of blood pressure to values of 56, 65, and 76 mm Hg, but no symptoms of cerebral hypoperfusion were elicited. CBF was unchanged after captopril administration, even in the patients showing a marked reduction in blood pressure. In the control group, the blood pressure, CBF and the cerebral metabolic rate for oxygen remained essentially constant following captopril administration. It is concluded that the cerebral circulation is well preserved during captopril treatment of chronic heart failure. This might be explained by a shift of the lower limit of CBF autoregulation towards lower blood pressure levels.