Continuous 72-h recordings of duodenojejunal contractile activity were obtained from 20 freely ambulant subjects; pressure was detected by two strain-gauge sensors incorporated in a transnasal catheter attached to an encoder and a miniature tape recorder. The subjects were 12 patients with irritable bowel syndrome, 6 of whom were constipation predominant and 6 of whom were diarrhea predominant, and 8 healthy controls. The procedure was well tolerated by all subjects and did not interfere with sleep or normal activity. In all subjects, the diurnal migrating motor complex cycle was characterized by a brief phase 1 and a prolonged phase 2; this was reversed during sleep when phase 2 was virtually absent. All subjects showed a circadian variation in migrating motor complex propagation velocity, and there was no difference in the patterns of motor activity during sleep between any of the groups. During the day, the duration of postprandial motor activity was shorter in irritable bowel syndrome patients than in controls, and diurnal migrating motor complex intervals were shorter in diarrhea-predominant than in constipation-predominant irritable bowel syndrome. In 11 of 12 inflammatory bowel syndrome patients, episodes of clustered contractions recurring at 0.9-min intervals were noted; these episodes had a mean duration of 46 min and were often associated with transient abdominal pain and discomfort. In both groups of irritable bowel syndrome patients, defecation was significantly (p less than 0.01) prolonged with a greater number of voluntary abdominal contractions (p less than 0.01) than in controls. Prolonged ambulant monitoring of proximal bowel motor activity in subjects who are free to move, eat, and sleep as they choose has, for the first time, clearly defined the striking difference in motility between the sleeping and waking state and shown that abnormalities associated with irritable bowel syndrome are confined to the latter.