BACKGROUND The aim of this study was to investigate the effects of intracolonic capsaicin on colonic motility and defecation. METHODS The effects of capsaicin (1, 2, 5, and 10 mg) administrated into the proximal colon on ileocolonic motility and defecation were studied in neurally intact dogs with or without various antagonists (atropine, hexamethonium, ondansetron, propranolol, and FK224), dogs with extrinsic denervation of an ileocolonic segment, and dogs with enterically isolated ileocolonic loops equipped with strain gauge force transducers. RESULTS Capsaicin at 5 and 10 mg evoked giant migrating contractions in a dose-independent manner, and it induced defecations with more than 90% probability in neurally intact dogs. These effects of capsaicin were abolished by atropine and hexamethonium. Ondansetron inhibited the capsaicin-induced increase in colonic motility but did not affect the induction of defecation. The other antagonists had no effect. In dogs with extrinsic denervation, capsaicin did not evoke giant migrating contractions in the colon but still induced defecation in 30-40% of experiments. In dogs with ileocolonic loops, capsaicin did not stimulate colonic motility nor induce defecation. CONCLUSIONS These results indicate that intracolonic capsaicin causes giant migrating contractions and defecation. Intact extrinsic innervation, continuity of the colon, and intraluminal contents were considered necessary for this effect.