From the comparative biochemical standpoint free amino acids and related substances have been isolated and quantatively analyzed from the brain of the common turtle (Geoclemys reevesii) in hibernation, the animal belonging to the reptile family, and the following are the results. 1. In comparison with those in the brain of mammals, aspartic acid, glutamic acid, alanine, serine, and glycine have been found only in extremely small quantities. 2. The quantity of both taurine and threonine has been less than either of them found in the brain of a catfish or a rat. 3. No isoleucine, leucine, lysine, nor histidine can be detected, and unidentified ninhydrin positive substances such as X(4), X(5), and X(6) have been recognized. 4. Despite the small quantity of glutamic acid, the amount of γ-aminobutyric acid has been about the same as found in the brains of mammals. 5. The total quantity of amino-N is less than that in the brain of catfish, frog, or rat.