In this review of brain energy metabolism, the utilization of glucose as the sole energy source in the normal brain is described and presented in a schematic form. Important reactions concerned with the synthesis and degradation of the energy carrier in the cell, ATP, are also noted. Changes in energy metabolism with acute oxygen deficiency are shown in the example of rat brain energy metabolism during 30 seconds of nitrogen breathing (significant decrease of phosphocreatine, ATP, total adenine nucleotides and adenylate energy charge). Several important metabolic changes, particularly in ion and lipid metabolism during the decrease in the cerebral blood flow (ischemia), are described in some detail and presented in schematic form. A short description of the [14C]-2-deoxyglucose method developed by L. Sokoloff for the determination of local cerebral utilization of glucose is followed by a description of the application of this method in the study of aging in rats.