The effects of enflurane anesthesia on cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRO2) were studied in 17 patients. The patients were divided into two groups according to the depth of anesthesia. Cerebral perfusion pressure was maintained above 60 mmHg with phenylephrine. In Group 1 (arterial enflurane concentration, 15 mg/dl), patients were studied before surgery, while in group 2 (enflurane concentration, 27 mg/dl), the measurements were performed before and during surgery. In Group 1, mean CBF and CMRO2 were 53 and 2.8 ml X 100 g-1 X min-1, respectively. These values were not significantly different from CBF (46 ml X 100 g-1 X min-1) and CMRO2 (3.1 ml X 100 g-1 X min-1) values previously obtained in awake patients. In Group 2 before surgery, mean CBF and CMRO2 were 61 and 2.6 ml X 100 g-1 X min-1, respectively, and were significantly different from the awake values, while the EEG showed frequent spikes and suppression. In Group 2 during surgery, mean CBF and CMRO2 did not differ from the values obtained before surgery, despite significant EEG changes. The results indicate that enflurane is a cerebral vasodilator and causes an increase in CBF and a decrease in CMRO2 in humans at an anesthetic level characterized by frequent spikes and suppression on the EEG.