Among patients complaining of constipation, a group can be defined in which there is slow whole gut transit shown by retention of radiopaque markers but a rectum and colon of normal width judged by measurements of barium enema radiographs compared with control observations. It is not known whether their symptoms are due to an abnormality of colonic motility or to a failure of the defecatory mechanism. Defecation was simulated experimentally in a group of these patients by asking them to expel a water-filled rectal balloon. The constipated patients were not able to expel the balloon, whereas normal subjects could do so. Electromyography of the striated pelvic floor muscles during attempts at expulsion of the balloon in the constipated patients showed failure of the normal inhibition of resting activity. Failure of external and sphincter relaxation on attempted defecation may contribute to the symptoms of some patients who complain of constipation.