The effect of moderate exercise on jejunal absorption was examined in seven healthy subjects using a triple lumen perfusion technique. Moderate exercise on a bicycle ergometer significantly reduced net absorption of water from 32.0 (4.0) to 16.2 (6.1) ml/30 cm/50 min (p less than 0.02), sodium from 2.4 (0.4) to 0.5 (0.9) mmol/30 cm/50 cm (p less than 0.05), chloride from 2.0 (0.4) to 0.3 (0.7) mmol/30 cm/50 min (p less than 0.05), and potassium from 0.20 (0.02) to 0.01 (0.04) mmol/30 cm/50 min (p less than 0.01). After exercise, water, sodium, and chloride absorption returned towards basal values, but potassium absorption remained significantly decreased. These results suggest that moderate exercise can influence jejunal absorption of salt and water in man. They support the possibility that the autonomic nervous system has a physiological role in the control of intestinal transport, although other mechanisms may be involved.