Ramosetron is a potent and selective serotonin (5-HT)₃ receptor antagonist that has been shown to affect abnormal colonic function and abdominal pain in animals. Ramosetron (0.3 to 100 µg/kg, p.o.) has been found to significantly suppress abnormal defecation induced by conditioned-fear stress (CFS), restraint stress, corticotropin releasing factor (CRF) and 5-HT in rats and mice, and these effects were more potent than those of alosetron, cilansetron or loperamide. On the other hand, ramosetron (3,000 µg/kg, p. o., once daily for 7 days) did not inhibit normal defecation in dogs while tiquizium significantly inhibited it. Ramosetron (3 to 100 µg/kg, p. o.) also significantly prevented CFS-induced acceleration of colonic transit and CRF-induced abnormal water transport in rats, respectively. Moreover, ramosetron (0.3 to 3 µg/kg, p. o.) significantly suppressed restraint stress-induced decrease in colonic pain threshold, an effect not observed with loperamide. These results indicate that ramosetron produce beneficial clinical effects on IBS symptoms.