High-fibre diet and laxatives are not always successful in the treatment of severe constipation. In a group of thirteen women with intractable constipation three experimental models of rectal evacuation showed that all had a disorder of defaecation. None could expel 120 ml barium paste as fast or as completely as could ten normal women; six could expel hardly any barium. Only three of the thirteen could expel a balloon containing 50 ml water from the rectum, whereas all of six normal subjects could do so easily. Four of the patients with repeated straining efforts could expel no more than 10% of a 500 ml saline enema, whereas six normal subjects rapidly passed 40-80%. It is postulated that dietary and drug treatment designed to soften and increase the bulk of the stools fails to help some patients because they find it difficult to expel even semi-solid or liquid stool. Electromyography showed that these patients tend to contract rather than relax the striated muscle of the pelvic floor on attempted defaecation. Their failure to defaecate is due to incoordination of the pelvic floor rather than an abnormality of the stool or a disorder of the colon.