Duplex ultrasound was used to investigate superior mesenteric artery haemodynamics in humans in order to study the contribution of the small intestine to the postprandial splanchnic hyperaemia, and to determine the relative potencies of the major food components in the postprandial mesenteric flow response. Duplex parameters of vessel diameter, mean velocity, and volume flow were determined serially in the basal state and after stimulation. Flow parameters were significantly (p less than 0.05) increased after liquid and solid oral meals. Modified sham feeding did not alter mesenteric blood flow. Intestinal perfusion of an isocaloric liquid test meal induced flow increases comparable with oral intake. Superior mesenteric artery blood flow also significantly (p less than 0.05) increased after isocaloric and iso-osmolar loads of intraduodenal carbohydrate, fat, and protein meals. Responses were similar after the test meal, fat, and protein, but were significantly (p less than 0.05) less for carbohydrates. Different osmolar loads of saline did not affect flow responses. We conclude that the intestinal phase is the major regulator of the postprandial mesenteric blood flow response in healthy humans and that the chemical nature of food determines the mesenteric response pattern.