The cerebral blood flow (CBF) and oxygen consumption (CMRO2) were determined in rats under superficial (N2O) anesthesia. The CBF was measured with a Xenon133modification of the Kety and Schmidt principle, using arteriovenous sampling during the desaturation of the tissue following a 20 min saturation period. The CMRO2 was calculated from the CBF and from the arteriovenous difference in total oxygen content. Since venous blood was sampled from the superior sagittal sinus the values for CBF and CMRO2 are probably representative of cortical tissue. Control experiments with measurement of Xenon133activities in arterial blood and in tissue, as well as in blood from different cranial veins, showed that the tissue under study contained no slowly perfused masses, and that significant contamination of superior sagittal sinus blood with blood from extracerebral sources did not occur. The results demonstrate that the blood flow of the rat cerebral cortex is 2–3 times higher, and the oxygen consumption 3–4 times higher, than the corresponding values for the human brain. These differences partly reflect a difference between cortical tissue and whole brain but an inverse relationship between body size and cerebral metabolic rate also seems involved.