This study examined the hypothesis that 5HT3 mechanisms mediate the postprandial gastrocolonic response in humans. Fasting and postprandial colonic tone and motility were studied in 12 healthy volunteers and the effects of a selective 5HT3 antagonist, ondansetron assessed in a double blind, randomised, placebo controlled fashion. A manometry barostat assembly was positioned in the transverse or descending colon to quantitate contractile activity fasting, after drug infusion and postprandially after a 1000 kcal meal. Fasting colonic tone and motility indices were similar in the placebo and ondansetron groups; ondansetron did not affect fasting motility. The placebo group showed a significant reduction in barostat balloon volume (signifying increased tone) from 232 ml (median, interquartile range (IQR) 179-261) during fasting to 181 ml (median, IQR 128-208) (postprandially) (p = 0.02). In contrast, the ondansetron group did not have a tonic colonic response (median 248 ml (IQR 199-300) fasting to median, 226 ml (IQR 185-290) postprandially) after the meal. Phasic volume events measured by the barostat increased postprandially in both groups. Postprandial motor activity measured by manometry increased significantly in the placebo group, but not in the ondansetron group. In conclusion, a 5HT3 mechanism participates in the physiological contractile responses in the human transverse and descending colon after ingestion of a high energy meal.