Analysis of fish brain for the dipeptide homocarnosine has led to the discovery of a new oligopeptide which is named neurosine. This substance contains γ-aminobutyric acid (GABA) and histidine residues. Most of the histidine present in a water soluble fraction from fish brain appears to be in the form of a tetrapeptide consisting of glycine, glutamic acid, GABA and histidine. Neurosine is ninhydrin negative, heat stable, and has a high Rfvalue when chromatographed in a butanol–acetic acid–water solvent system. The evidence at present suggests that this compound may be a cyclopeptide or contains a pyrrolidone or diketopiperazine ring structure.Neurosine is present in high concentration in the brain of the killifish,Fundulus heteroclitus, approximately 0.1–1.0 μg of peptide per milligram of tissue. It also has been found in other bony fish, amphibians and reptiles but is absent in cyclostomes, elasmobranchs, birds and mammals. Neurosine is not present in any tissue but brain.