A subpopulation of reticulospinal neurons in the posterior rhombencephalic reticular nucleus in lamprey contains a cholecystokinin (CCK)-like peptide. Varicosities of these neurons, visualized by immunohistofluorescence, were found to be in close apposition to Lucifer yellow-filled spinal motoneurons and giant relay interneurons. Certain other types of interneurons and primary sensory neurons had no, or very few, close contacts with CCK-immunoreactive boutons.