AbstractCerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRo2) were measured in rats under nitrous oxide anaesthesia, using a133Xenon modification of the Kety and Schmidt inert gas technique with sampling of cerebral venous blood from the retroglenoid vein. Extracerebral contamination of the venous blood sampled was studied by comparing the rates at which the activity of133Xenon decreased in blood and tissues. Contamination was avoided by gentle compression of the contralateral retroglenoid vein during sampling. CBF and CMRo2of the rat brain were 80±2 and 7.6 ± 0.2 ml·(100 g)‐1, min‐1, respectively. These values are about 25% lower than those previously obtained for cerebral cortical tissue under similar conditions. Induced hypercapnia (Paco2about 70 mm Hg) or hypocapnia (Paco215–20 mm Hg) gave rise to expected changes in CBF but did not alter CMRo2The CMRo2of the rat brain is at least twice that of the human brain. This species difference, which is similar to that previously reported for the oxygen uptake of cerebral tissuein vitro, probably reflects on inverse relationship between brain weight and neuronal packing