Although bombesin (BN) regulates colonic motility, the associated mechanism of action remains unclear. The objective of this study was to investigate the effect of BN on colonic motility using isolated rat colon. An isolated rat colon was perfused with Krebs solution via the superior mesenteric artery. Intraluminal pressure was measured via microtip catheter pressure transducers at both proximal and distal portions of the isolated colon. After a control period, BN was administered intraarterially in concentrations of 13, 26, 130, and 260 pM. After pretreatment with hexamethonium (10-3 M), atropine (10-5 M), or tetrodotoxin (10-6 M), BN was administered at a concentration of 260 pM (proximal colon) or 130 pM (distal colon); intraluminal pressure was then monitored. Changes in motility were expressed as the percentage change of motility index (MI) over the basal period. As a result, BN increased colonic motility and a dose-dependent increase in proximal colonic motility was observed. The stimulant effect of BN was almost completely abolished by atropine and tetrodotoxin at both the proximal and distal colon. However, BN was not inhibited by hexamethonium at both the proximal and distal colon. Therefore, the stimulant action of BN may be mediated by local cholinergic muscarinic receptors.