Some patients with diarrhoea predominant irritable bowel syndrome have increased rectal sensitivity. It is uncertain, however, whether the diarrhoea is a consequence of the rectal sensitivity or if it is sensitising the rectum in some way. The aim of this study was to assess whether inducing diarrhoea in normal healthy volunteers can sensitise the rectum and therefore be a potential or partial cause of the sensitive rectum seen in some patients with diarrhoea predominant irritable bowel syndrome. The anorectal responses to balloon distension were measured in 20 healthy volunteers (aged 20-43 years, 10 female) eight hours after laxative induced diarrhoea or under control conditions. Ingestion of an isoosmotic laxative increased stool output from 1.1 (0.7-2.3) (median (range)) to 8 (5-19) bowel movements per day with no significant differences between men and women. In women rectal sensitivity was significantly increased after diarrhoea compared with control conditions (vol to induce discomfort (ml): 116 (96, 136) v 153 (137, 168), mean (95% CI); p <0.001). This was associated with a reduction in the volume to induce internal anal sphincter relaxation (16 (12, 20) v 28 (21, 36); p <0.005), and volume to induce sustained internal anal sphincter relaxation (70 (56, 84) v 90 (67, 113); p <0.03), but no significant change in rectal compliance (ml/cm H2O at 100 ml) 4.8 (3.5, 6.1) v 4.1 (3.0, 5.1) or distension induced motility (motility index) 994 (341, 1647) v 735 (46, 1424). Conversely, in men diarrhoea had no significant effect on anorectal physiology and their control values were not significantly different from those of the women. In conclusion, the results of this study taken with the finding that irritable bowel syndrome is more common in women, suggests that the male or female sex hormonal environment may be an important factor in allowing the gut to be sensitised to noxious stimuli.