Abstract A basic amino acid, putreanine, was found in mammalian brain but not in other organs. It was purified from bovine brain by ion exchange chromatography and was crystallized as the sulfate. From 82 kg of brain, 535 mg of the sulfate were obtained. Elementary analysis gave its empirical formula as C7H16N2O2. Its structure was subsequently determined as N-(4-aminobutyl)-3-aminopropionic acid by Shiba and Kaneko (see "Appendix"). The concentration of putreanine was determined by preliminary purification of the basic amino acid fraction from a trichloracetic acid extract of mammalian tissues followed by analysis in an automatic amino acid analyzer. It was only detected in the central nervous system of mammalian and avian organisms. Its concentration was highest in caudal regions of the brain, and the white matter of the cerebral and cerebellar cortices and the spinal cord contained more than grey matter of the corresponding areas. Putreanine appears in the brain of rats 2 weeks after birth and subsequently its concentration increases for several months.