The effect of hypoxia on the regional cerebral metabolic rate for glucose (rCMRgl) was measured in 28 neuroanatomical structures of adult, conscious, unrestrained rats by the 2-[14C]deoxyglucose autoradiographic technique. Rats were cannulated in one femoral artery and vein 3 days before the experiments. The rCMRgl was measured in 15 rats during 30 min air breathing and in 13 rats during 30 min hypoxia of 7% O2 in N2. Statistically significant increases in rCMRgl in the order of 39-95% were observed in 25 of the 28 neuroanatomical structures examined. The highest increases in rCMRgl were observed in cerebral and cerebellar white matter (95% and 60%, respectively), as well as in limbic structures ranging from a 91% increase in the septal nuclei to a 55% increase in the hypothalamus. The superior olivary nucleus and inferior colliculus (auditory structures) were the only structures which did not show changes in glucose utilization. The present data are compared with previous studies during hypoxia in conscious or anesthetized animals. It is concluded that the degree of rCMRgl response to hypoxia is affected by anesthesia, age and species.