With the aid of the chromatography on the 150 x 0.9 cm column of Dowex 50-x4 the author estimated 20 kinds of free amino acids and their related compounds isolated from the brain of the hypophysectomized Rattus; and obtained the following results: 1. A marked decrease has been observed in the metabolically active amino acid groups such as aspartic acid, glutamic acid, γ-aminobutyric acid, alanine and serine. 2. Likewise a decrease can clearly be noticed in the content of N-acetylaspartic acid that exists specifically in the brain and in the content of phosphor containing amine, glycerophosphoethanolamine as well as in the glutathione content, the only peptide which has been successfully estimated. 3. In the sutdy of 6 indispensable amino acids such as threonine, isoleucine, leucine, lysine, histidine, and arginine no noteworthy changes can be recognized.