The effect of exercise on large intestinal function has been determined in 14 healthy but normally sedentary men and women, aged 22-34 yr while on a constant diet. For an initial 3-5-wk period (control) no activity was allowed. Six subjects then undertook a 9-wk training schedule by the end of which they were capable of jogging for 1 h per day, 5 days a week. A further 6 subjects undertook a similar training schedule that lasted for only 7 wk, at the end of which they were jogging for 45 min per day. Finally, 2 subjects were studied continuously while taking light exercise for 6 wk and then jogging for a further 3 wk. Physical fitness was monitored and showed significant changes with maximum aerobic capacity increasing from 2.4 +/- 0.5 to 3.1 +/- 0.4 L/min, maximum heart rate after a step test falling from 152 +/- 8 to 129 +/- 5 beats per minute, and resting pulse rate also falling from 56 +/- 4 to 50 +/- 5 beats per minute. High-density lipoprotein cholesterol also increased significantly. Colonic function was assessed by measurement of stool weight and transit time, using the continuous radiopaque marker technique, fecal pH, nitrogen excretion, and ammonia concentration. No change was observed overall in mean daily fecal weight [124 +/- 39 (control) and 129 +/- 49 g/day (exercise)], transit time [55 +/- 20 (control), 54 +/- 23 h (exercise)], nor in fecal frequency, dry stool weight, pH, ammonia, or total nitrogen excretion. Significant changes did occur in 5 individuals with significant slowing of transit time in 2 and speeding up in 3. Overall transit time increased in 9 subjects and decreased in 5; hence, when diet is constant, exercise has marked effects on physical fitness but no consistent effect on large bowel function.