Summary The pattern of free amino acids in mouse brain, as shown by 2-dimensional paper chromatography, does not reflect the amino acid composition of the brain as a whole. The brains of mice of several strains and various parts of the nervous system of the rabbit were studied. Free glutamic and aspartic acids, γ-aminoibutyric acid, taurine, glutamine, cystine, serine, glycine, alanine, valine, and the leucines were detected consistently. The variations in these constituents with age, sex, and strain in mice are discussed. The sciatic nerve in the rabbit had considerably smaller quantities of detectable constituents than the various parts of the central nervous system. A transplantable neuroblastoma in mice had a pattern of free amino acids which was similar to that found in other malignant tissues and in brains of 15-day embryos and significantly different from that found in all of the other samples of mouse brain examined. The suitability of the chromatographic method for the study of changes in the free amino acids of brain homogenates is indicated.