1. In the tied off colon of the anaesthetized rat in situ, the effect of diphenolic laxatives (bisacodyl and phenolphthalein) and of osmotic laxatives (mannitol, sodium sulfate and lactulose) on water net flux was studied. All the laxatives reduced water net flux from lumen to blood or reversed it into water net flux from blood to lumen. Pretreatment with indomethacin reduced or abolished the effect of the diphenolic laxatives on water net flux but did not change the effect of the osmotic laxatives. 2. In the perfused colon of the rat in situ, the effect of the diphenolic and the osmotic laxatives on water net flux and on PGE-release into the colonic lumen was investigated. All laxatives reduced water net flux from lumen to blood in this preparation, too. PGE-release into the lumen during perfusion with osmotic laxatives was not different from the control. The diphenolic lexatives increased PGE-release dosedependently about 2-fold. 3. Intraluminal perfusion of the jejunal loop of the rat in situ with bisacodyl increased intestinal blood flow by about 50%. Addition of indomethacin to the perfusion fluid abolished this increase. 4. It is concluded that diphenolic laxatives exert their laxative action at least partially via stimulation of intestinal PGE-biosynthesis.