Neuromedin U-8 induced a monophasic and concentration-dependent contraction of intact small intestine from the turtle, Pseudemys scripta, whereas the peptide had no effect upon the motility of rat and guinea pig gut. The maximum response produced by neuromedin U-8 was 56% of that produced by acetylcholine and 62% of that produced by potassium chloride. The potency and maximum response to neuromedin U-8 were unaffected by tetrodotoxin and atropine. The data suggest that neuromedin U may play a role in regulation of gut motility in lower vertebrates but not in mammals.