Six patients with watery postvagotomy diarrhoea and 4 patients with intractable diabetic diarrhoea were treated with cholestyramine. The diarrhoea responded completely to therapy, during which the stool content of water, sodium, postassium and magnesium fell. Two of the patients, while receiving cholestyramine, had a rise in faecal fat, but this was not accompanied by diarrhoea, the stools being well formed. Evidence is presented which suggests that the mechanism of diarrhoea is similar in both disorders and is due to division or neuropathy of the hepatic fibres of the vagus nerve. This results is distension of the gallbladder, contraction of which expels increased quantities of bile salts, which swamp the reabsorptive capacity of the small intestine and induce diarrhoea by direct action on the colon. If this theory is valid, cholecystectomy, by preventing large quantities of concentrated bile salts from suddenly entering the gut, may result in improvement of postvagotomy and diabetic diarrhoea.