Rats were exposed to mild hyperoxia and hypoxia by the administration of oxygen/air and nitrogen/air mixtures through plastic tubing held close to their snouts for periods of 3 min. Changes in tissue oxygen were monitored at an implanted carbon paste electrode; local cerebral blood flow (rCBF) at an implanted platinum electrode using the hydrogen clearance technique; extracellular brain glucose at an implanted glucose oxidase-based biosensor and changes in lactate were measured using microdialysis. The nitrogen/air mixture led to a decrease in tissue oxygen, an increase in rCBF, a decrease in extracellular glcuose, and an increase in lactate. The oxygen/air mixture led to an increase in tissue oxygen and extracellular glucose but no change in lactate or rCBF. The effects in unanaesthetised rats were compared with those in rats given 350 mg/kg cloral hydrate. The increase in lactate was greater in unanaesthetised than anaesthetised rats. The results show a dissociation between changes in rCBF and extracellular glucose and suggest that changes in oxygen affect utilisation rather than supply of glucose to the extracellular compartment.