Large Amounts of cholecystokinin-octapeptide (CCK) are present in the rat caudatoputamen. The peptide occurs in axons and nerve endings but not in perikarya. The origin of CCK in the caudatoputamen was investigated with the use of immunocytochemistry and a radioimmunoassay specific for CCK. Although a small amount of CCK (approximately 30 percent) originates in the amygdaloid complex, the bulk of the peptide (approximately 70 percent) occurs in processes of neurons located ventral to the caudatoputamen, that is, the claustrum or the piriform cortex. The claustrum and piriform cortex receive inputs from various cortical areas and the olfactory system, respectively, and may process information and relay it to the caudatoputamen. Thus CCK may by the transmitter in the final common pathway linking various cortical areas and the olfactory system to the caudatoputamen.