The distribution of cholecystokinin-like immunoreactive (CCK-LI) neurons has been mapped in the rat medulla after local and intracerebroventricular colchicine injections. CCK-positive neurons were found in the nucleus raphe magnus, nucleus raphe pallidus, nucleus raphe obscurus, nucleus paragigantocellularis pars alpha, and a population of ventral medullary neurons. Combined retrograde tracing with the fluorescent dye True Blue and indirect immunofluorescence for visualizing CCK neurons suggested that there was a CCK-LI system originating in the medulla and projecting to the spinal cord. Additional double labelling experiments established that some of these CCK-LI containing neurons also contain 5-HT.