The regional enzyme activities of glucose metabolism in the rat brain were investigated. Hexokinase (EC 184.108.40.206) and pyruvate dehydrogenase (EC 220.127.116.11), key enzymes for glucose metabolism, showed no changes in activity in all the regions studied of the aging brain as compared with the adult brain. However, the activity of D-3-hydroxybutyrate dehydrogenase (EC 18.104.22.168) is low throughout the adult brain and, in contrast with hexokinase and pyruvate dehydrogenase, its activity decreases significantly during aging. Other enzymes that showed significant decreases during aging are aldolase (EC 22.214.171.124), lactate dehydrogenase (EC 126.96.36.199), citrate synthase (EC 188.8.131.52), and NAD+-linked isocitrate dehydrogenase (EC 184.108.40.206). The catabolic enzyme in cholinergic metabolism, acetylcholinesterase (EC 220.127.116.11), selected as an example of a non-energy-metabolising enzyme, also showed significant decreases in all regions of the brain in aging, although its highest activity remained in the striatum. These results are discussed with respect to the energy metabolism in various brain regions and their status with aging.