Cerebral blood flow has been measured in dogs using the method of Lassen and Ingvar (1961), and Ingvar and Lassen (1962), which involves measurement of the rate of clearance of krypton 85 from an area of exposed cerebral cortex after injection into the internal carotid artery. In every measurement during halothane anaesthesia there was a considerable reduction in cerebral blood flow compared with the flows obtained with nitrous oxide and oxygen. The average reduction in flow was 46 per cent and this was accompanied by an average increase in cerebrovascular resistance of 50 per cent. Measurements were also made of the cerebral oxygen uptake (CMRO2) and halothane produced an average decrease in uptake of 49 per cent. Possible mechanisms for the observed results are discussed.