FMRFamide-like immunoreactivity in cells and fibers of the holothurian nervous system. Immunoreactivity to FMRFamide (FMRFa) was detected in several components of the nervous system of the sea cucumber, Holothuria glaberrima. Neurons and fibers expressing immunoreactivity to this peptide were found in the radial nerves, and in nerve plexuses of the esophagus, and large and small intestine. The neurons in the enteric nervous system were located in the serosal layer and immunoreactive fibers appeared to innervate the longitudinal muscle. Co-existence of immunoreactivities to FMRFa and cholecystokinin was detected in most of the enteric fibers. Therefore, in the holothurians, neurons expressing FMRFa and FMRFa/cholecystokinin might be involved in the physiology of the digestive tract.