A series of neuromedin U (NmU)-like peptides were found to evoke concentration-response contraction of rat fundic circular muscle in vitro. Isometric contraction of this tissue was induced by rat NmU, porcine NmU-8 and NmU-25, and frog NmU. Rat NmU was significantly more potent than frog NmU. The contractile action of rat NmU upon rat fundic strips was not affected by either atropine or tetrodotoxin indicating a direct effect. Maximal NmU-induced contractions were 10% of the maximal contraction induced by carbachol and ranged between those of the bradykinin and angiotensin II. None of the NmU peptides were active on the circular smooth muscle of the frog stomach or on the small and large intestinal longitudinal smooth muscle of either rat or frog. The results of this study demonstrate that members of the NmU peptide family all induce in vitro contraction of rat gastric circular smooth muscle independent of cholinergic or other neuronal mechanisms. Their activity, however, appears to be both tissue and species specific.