The cerebral metabolic and vascular effects of isoflurane (Forane) were investigated in six unmedicated ventilated dogs. At the MAC of this anesthetic (1.4 per cent, end-expired) there was a 23 per cent decrease in the rate of cerebral oxygen consumption (CMRO2) (compared with values at end-expired concentrations of <0.1 per cent). At a higher concentration of isoflurane (2.4 per cent, end-expired), a 30 per cent reduction in CMRO2, was observed. Cerebral blood flow (CBF) increased by 33 and 63 per cent at the 1.4 and 2.4 per cent concentrations, respectively. The increase in CBF was due entirely to a decrease in cerebral vascular resistance (CVR) and occurred despite an accompanying significant decrease in arterial blood pressure. The response of CBF to change in PaCO2 was appropriate during isoflurane anesthesia and was not different from that previously observed during halothane and metboxyflurane anesthesia.