Glutamate (glu) is the most abundant excitatory neurotransmitter in the central nervous system and astrocytic clearance of glu is essential for avoidance of glu induced excitotoxicity. Continuance of astrocytic glu removal relies upon metabolism of glu within the astrocyte. Glu entering the astrocytes may either be amidated to glutamine as part of the glu-glutamine cycle or metabolized in the mitochondrial matrix to alpha-ketoglutarate (aKG). The conversion to aKG is catalyzed either by glu dehydrogenase (GDH) or a transaminase. The GDH catalyzed deamination of glu is an energy producing reaction and it also provides an increased pool of TCA cycle intermediates. Thus, GDH is positioned at an intriguing and important place on the metabolic chart having potential impact on both brain energy and glu homeostasis. To investigate the role of GDH in the brain, a CNS specific GDH knock-out (KO) mouse has been constructed. Experiments using cultured astrocytes from the GDH brain KO mice and “wild type” littermates have been conducted. Cytosolic ATP production was measured on-line in living astrocyte cultures infected with luciferase. The ATP production was monitored upon glu and glucose (glc) exposure. The GDH brain KO failed to respond to glu but exhibited an increased ATP production after glc addition. In line with this the metabolism of [U-13C]glc was augmented in the GDH brain KO astrocyte cultures. Thus we find that GDH is essential to sustain the energy status in astrocytes.