The influence of moderately reduced arterial oxygen tension (aPO2 of about 45 Torr) on the metabolism and the blood flow of the brain was tested in 20 anaesthetized, artificially ventilated normotensive, normocapnic beagle dogs. It is demonstrated that the decrease in systemic oxygen delivery to the brain is countered by an appropriate increase in flow (CBF being 60.3 ml/100 g min at normoxia and 84.5 mg/100 g min in hypoxaemia) which maintained the cerebral oxygen consumption unchanged (CMRO2 3.80 versus 3.32 ml/100 g min). The cortical tissue content of energy-rich phosphates such as ATP, ADP, AMP, and phosphocreatine was also found to be unaltered. Neuropathological examinations excluded any hypoxic cell damage. This reactive vasodilatory reaction of the cerebral vessels is apparently a sensitive regulatory process which protects the brain against marked oxygen lack. However, a normal carbohydrate metabolism is not restored by this cerebrovascular mechanism. For, significantly increased CMRlactate (0.32 versus 1.46 ml/100 g min) indicated raised cerebral glycolysis, and the tissue metabolites of glucose suggested an increased glycolytic flux in the brain. It is concluded that in moderate arterial hypoxaemia, which is not uncommon in clinical practice, cerebral blood flow plays an effective homeostatic role in preventing a disturbance of the energy metabolism of the brain.