The effect of granisetron, a specific 5-hydroxytryptamine 3-receptor antagonist, on the anorectal responses to rectal distension and a 1000-calorie meal was assessed in 12 patients with irritable bowel syndrome. Each patient was studied on three occasions, receiving intravenously either 40 mcg/kg granisetron, 160 mcg/kg granisetron or normal saline. Granisetron caused a dose-dependent reduction in rectal sensitivity, manifested by an increase in the threshold volumes at which the sensations of gas, desire to defecate, urgency and discomfort were perceived. This reached significance for all sensations at the higher dose level (P < 0.01). No significant changes in anal pressures, rectal compliance or distension-induced motor activity occurred following drug administration. A dose-dependent reduction in post-prandial motility was observed following intravenous granisetron and this was highly significant at 160 mcg/kg (P = 0.005). These results suggest that the 5 hydroxytryptamine receptor antagonists may have a therapeutic role in patients with irritable bowel syndrome.