The effects of castor oil, alone, as well as in combination with PGI2 and indomethacin on gastrointestinal functions have been examined in rats. Oral administration of the oil to fasted rats induced severe diarrhoea, with increased intestinal motility and fluid volume. Pretreatment with PGI2 (s.c.) inhibited the effect of the oil on intestinal fluid accumulation and decreased intestinal motility below control values, but only delayed the occurrence of mucoid diarrhoea. Indomethacin (i.p.) reduced the accumulation in intestinal fluid after castor oil administration to a much smaller extent (47%) than PGI2 and depressed the increased intestinal motility to control values. In contrast to PGI2, indomethacin inhibited the occurrence of diarrhoea after administration of castor oil. The present results do not definitely confirm the general opinion that the diarrhoeal action of laxative agents is due only to an altered intestinal electrolyte and water transport or an increase of intestinal motility.