OBJECTIVE To examine how moderate exercise affects the blood flow in the gastrointestinal tract. METHODS Twelve miniature swine weighing 38-43 kg were used. All animals were trained on a cardiac exercise treadmill. Blood flow measurements were done on conscious animals using labeled microspheres with a diameter of 16.5 +/- 0.1 (SD) microm. The first flow was measured while the animal was awake and resting, the second flow after 15 min of exercise, the third flow after 30 min of rest. RESULTS Flow in the oesophagus at rest was 19.5 +/- 1.3 (SEM) ml/min/100 g. During exercise the flow decreased to 13.3 +/- 1.2 (SEM) ml/min/100 g (ns). After 30 min of rest the flow was 9.9 +/- 1.2 (SEM) ml/min/100 g (p < 0.05 when comparing the flow before and after exercise). Flow in the cardia at rest was 23.1 +/- 1.3 (SEM) ml/min/100 g. During exercise the flow decreased to 14.0 +/- 1.2 (SEM) ml/min/100 g (p < 0.05). After 30 min of rest the flow was 15.0 +/- 1.2 (SEM) ml/min/100 g. Flow in the pylorus at rest was 38.9 +/- 1.1 (SEM) ml/min/100 g. During exercise the flow decreased to 24.6 +/- 1.1 (SEM) ml/min/100 g (p < 0.01). After 30 min of rest the flow was 26.9 +/- 1.2 (SEM) ml/min/100 g. Blood flow in the small and large intestine was mainly unaffected by moderate exercise. CONCLUSIONS Under moderate exercise, blood flow in the upper part of the gastrointestinal tract declines while it is mainly unaffected in the duodenum, small and large intestine.