We measured the effect of exercise, at different intensities, on duodenojejunal motor activity in eight trained, ambulant cyclists, using a catheter with two strain gauge transducers connected to a solid-state data logger. Orocecal transit time was measured by H2 breath test. Subjects were studied for two consecutive days and fed twice a day with a similar, 860-kcal meal. Control recordings on the first day were made at rest and a naloxone infusion was given after the evening meal. On the second day, the athletes exercised after meals for 20 min at 80% of peak O2 uptake (VO2max) and naloxone was infused during the second session. On separate days, five subjects exercised for 3 h at 60% VO2max and for 10 min at 90% VO2max. Plasma concentrations of beta-endorphin were determined before and after exercise. The fed pattern was interrupted by an activity front during 2 of 16 sessions of exercise at 80% VO2max, in the presence of naloxone, and 5 of 8 sessions at 90% VO2max, with and without naloxone. It was not affected by exercise at 60% VO2max. Orocecal transit was not affected by intense exercise. The results show that exercise can affect intestinal postprandial motor activity. The effect is intensity related and may not be mediated by opioids.