NGIWYamide is a peptide recently isolated from the sea cucumber Apostichopus japonicus. It sti¡ens the connective tissue of the holothurian body wall. Localization of NGIWYamide was investigated by immunohistochemical staining with antiserum raised against NGIWYamide. In holothurian nervous systems NGIWYamide-like immunoreactivity (NGIWYa-LI) was observed in the hyponeural and ectoneural regions of the radial nerve cord, as well as in the circumoral nerve ring, podial nerves, tentacular nerves, the basiepithelial nerve plexus of the intestine and in cellular processes running through the body wall dermis. Labelled nerve ¢bres from the hyponeural part of the radial nerve running towards the circular muscle and from the podial nerve into the body wall dermis suggest that NGIWYamide controls both muscle and connective tissue. We examined the e¡ect on muscle activity of the sea cucumber. NGIWYamide (10 77 to 10 74 M) caused contraction of the longitudinal body wall muscle. Tentacles showed contraction only at a higher dose (10 74 M). NGIWYamide (10 74 M) inhibited spontaneous contraction of the intestine.