A transcutaneous Doppler ultrasound method was used to measure the superior mesenteric artery blood flow in nine healthy volunteers in the fasting state and serially for 1 h after the ingestion of 400 ml of an isotonic glucose solution. These measurements were repeated on a second occasion following the ingestion of 400 ml of an isotonic lactulose solution. Superior mesenteric artery blood flow increased by 53 per cent (p less than 0.05) 5 min after the end of ingestion of the glucose solution. The increase persisted at 10 min and declined to 47 per cent at 15 min and to 23 per cent at 30 min. No significant change in blood flow was found following the ingestion of the lactulose solution. Significant differences between the two responses were found at 5, 10 and 15 min (p less than 0.05). The increase in blood flow after a glucose solution and not after lactulose suggests therefore that the process of absorption is an important factor governing postprandial superior mesenteric artery blood flow.