Cholecystokinin carboxyterminal octa- and tetrapeptide concentrations have been measured in rat brain by a combination of high pressure liquid chromatography and radioimmunoassay. The sulfated octapeptide is the predominant form of Cholecystokinin in rat brain (approx. 80%). The concentration of the tetrapeptide represents 5-10% of that of the sulfated octapeptide in molar terms, depending upon the brain region. In addition to the tetrapeptide, a peptide with the properties of Cholecystokinin octapeptide in its nonsulfated form could be detected in concentrations similar to those of the tetrapeptide.