A combined manometric and radioisotope study is described which was designed to investigate the motility characteristics of induced defaecation in the colon, rectum and anal canal, in patients with severe constipation. Five women with severe idiopathic constipation, aged 30-45 years, were studied. A multiple channel water-perfused manometric tube was placed colonoscopically into either the whole colon or left colon to monitor pressure activity. In two whole-colon studies, 5.5 MBq 111In DTPA and later 9 MBq 99mTc DTPA were instilled into the colon to monitor the transit of colonic contents in response to two separate stimuli. Motor activity was initiated on both occasions by intraluminal instillation of 5.5 mg bisacodyl. In the whole-colonic studies a right colonic stimulus initiated multiple high-pressure peristaltic waves which traversed the entire length of the colon. The bowel was then refractory to a second stimulus at the splenic flexure. In the five left colonic studies, a stimulus at the splenic flexure produced high-pressure peristaltic waves which proceeded distally, and which included a rise in rectal pressure. The anal canal relaxed simultaneously with the onset of peristalsis high in the left colon, and remained relaxed until the pressure wave reached the anus and bowel content had been expelled. In both types of study high-pressure waves corresponded to the effective transport of radioisotope. The constipated patients showed a coordinated motor response to the stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)