The present investigation was designed to study the role of stress on the physiologic mechanisms of the colon in irritable bowel syndrome (IBS). Patients with IBS were compared with normal controls during resting and stress (mental arithmetic, cold pressor, and fear stressor). The results indicated that IBS patients had significantly higher motor activity than normals in the resting state but did not differ from them in the mean dominant frequency of the basal electrical rhythm (BER) or the proportion of the time they had 2-4 cycles per minute (cpm) slow-wave activity. Stress significantly increased motor activity in both groups although they did not differ significantly from each other during stress. Stress increased the proportion of 2-4 cpm slow-wave activity in IBS patients, but decreased in the controls. The type of stressor, however, did not influence either motor or electrical activity. Although IBS patients were significantly older than the controls and scored higher on the MMPI scales of Hypochondriasis, Hysteria, and Depression, these factors did not significantly influence differences in motor or electrical activity between the groups. The results are discussed in terms of the role of learning in the colon.