The abdominal distribution of orally ingested radioopaque markers is used to assess total and segmental colonic transit in constipated patients, but interpretation may depend on whether studies are carried out on a full colon or one cleared of faeces. We asked 25 severely constipated patients (age 18-74; 22 F, 3 M) to ingest 50 polyethylene markers (4 mm x 2 mm) at breakfast on 2 occasions 1 month apart. No bowel preparation was used for study 1 but for the second study 2 doses of Sodium Picosulphate (2 x 10 mg) were taken 3 days before ingestion of the markers. All subjects reported a good result which had ceased a day before taking the markers. Marker distribution was assessed by a plain abdominal film taken 72 hours after ingestion. All 25 subjects had more than 50% of markers present at 72 h in study one. Of these 4 showed evidence of outlet obstruction with more than 50% of ingested markers in the rectum. In the remaining 21 subjects markers were distributed throughout the colon in a pattern indicating colonic inertia. Following the administration of purgative there was no significant change in the mean number of markers retained but patterns of marker distribution for individual subjects did alter. Of the 21 patients who previously had colonic inertia, 3 showed outlet obstruction and 1 showed no evidence of delayed transit. Of the 4 patients with a pattern of outlet obstruction in the first study, 2 showed no evidence of delayed transit after Picolax.(ABSTRACT TRUNCATED AT 250 WORDS)