Transcutaneous Doppler ultrasound was used to measure superior mesenteric artery blood flow in 12 healthy volunteers in the fasting state and serially for 1 h after the ingestion of isocaloric and isovolaemic carbohydrate, fat, and protein liquid meals. The superior mesenteric artery blood flow increased significantly within 5 min of the end of each meal. The maximal responses were not significantly different but were reached at different times: carbohydrate, 64% at 15 min; fat, 60% at 30 min; and protein, 57% at 45 min. The response to the fat meal was significantly slower than the response to the carbohydrate, and the response to protein was slowest of all. In a further group of 20 fasted normal subjects no significant change in superior mesenteric artery blood flow occurred after drinking 400 ml of distilled water at room temperature. In seven of these subjects, drinking 400 ml of distilled water at 4 degrees C also did not affect mesenteric blood flow. These results indicate that the chemical nature of the meal and not the volume per se is a significant factor determining postprandial mesenteric hyperaemia.