Cerebral blood flow and cerebral oxygen uptake were studied during severe arterial hypoxia in anesthetized dogs. It was shown that the hypoxic vasodilatation in the brain reaches a limit at an arterial oxygen saturation at about 25% and that this vasodilatation is less than that which may be induced by hypercapnia. A further deepening of the arterial hypoxia at a maintained cerebral perfusion pressure is combined with a continuous decrease in cerebral venous oxygen tension and a reduced oxygen uptake.