The involvement of 5-hydroxytryptamine3 (5-HT3) receptors in gastric motor and sensory responses to distension and duodenal lipid was investigated. Subjects were studied on four occasions during which isotonic saline or 20% Intralipid (2 kcal/ml) was infused intraduodenally (1 ml/min) while the proximal stomach was distended with air (100 ml/min). Subjects received either 8 mg ondansetron (5-HT3 antagonist) or placebo orally in random order. Intragastric pressure was recorded continuously, and subjects reported gastric sensations. Gastric motor and sensory responses to distension during duodenal saline were similar with placebo and ondansetron. Intraduodenal lipid decreased gastric tonic and phasic pressure activity, and this was not influenced by ondansetron. Lipid also induced meal-like fullness followed by nausea during distensions. Ondansetron reduced nausea and did not affect meal-like fullness but increased volumes and pressures at which sensations were reported. Intestinal 5-HT3 receptors are involved in induction of nausea but not of meal-like fullness by intraduodenal lipid and gastric distension. 5-HT3 receptor antagonism reduces gastric sensitivity to distension during intraduodenal lipid infusion.