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Extracellular (EC) concentrations of amino acids were determined in the rat dentate gyrus by means of non-linear regression analysis of ‘in vivo’ brain dialysis data, considering a simple model of diffusion through a dialysis membrane. The apparent diffusion constants (K) of several amino acids were also calculated in the ‘in vivo’ situation. While putative amino acid neurotransmitters (glutamate, aspartate and γ-aminobutyric acid (GABA) were present in the EC fluid at the low micromolar range (0.8–2.9 μM), glutamine was by far the most prominent (193.4 μM). The values of intra/extracellular concentration ratios formed 3 groups: (a) high (>2000) for putative neurotransmitters; (b) low (<100) for serine, glutamine, arginine and α-alanine; and (c) intermediate (about 400) for taurine. The ‘in vivo’ calculatedK values proved useful for estimation of both basal and changing EC concentrations of amino acids in relatively …