Glucose is the major fuel of the adult brain under normal conditions. The most abundant free amino acids of the brain are: Glutamate, taurine N-acetylaspartate, glutamine, aspartate and 4-aminobutyrate. Deprivation of glucose from the incubation medium of brain cortex slices decreases mainly glutamate and glutamine content. Substitution of pyruvate for glucose maintains the content of aspartate and glutamate but not of glutamine. Therefore the entire glucose molecule is indispensable for the maintenance of the glutamine content. When branched chain amino acids, (BCAA), leucine, isoleucine and valine (0.5 mM) are added to the incubation medium in the presence of glucose the content of glutamine only is increased, all other amino acids are decreased. This suggests that BCAA spare protein as in muscle and produce glutamine. Branched chain amino acids in the presence of pyruvate do not spare protein but they still produce glutamine. Furthermore it seems that they potentiate the protein sparing effect of glucose in brain. In all these experiments glutamate and aspartate content in the incubation medium ranges from 3-10% of their total content (tissue plus media). On the other hand 40-60% of each of the other amino acids is found outside. The presence of 3-hydroxybutyrate and glucose in the incubation medium of brain cortex slices from normal rats spares protein but can maintain constant the content of glutamate, aspartate and glutamine. In addition 3-hydroxybutyrate blocks the release into the medium of glutamate and aspartate.