Magnesium sulphate, a substance known to cause release of cholecystokinin (CCK) from the small intestinal mucosa, was given by mouth (dose 0.1g/kg in 150 ml water) to 20 patients with the irritable bowel syndrome. A rapid increase in colonic segmental motor activity (onset within two to six minutes in most cases) was seen (percentage activity increased from 16.2 to 23.7 p<0.05; mean wave amplitude from 7.1 to 9.1 cm H(2)O, NS; motility index from 144 to 259, p<0.01). This increase was most marked in 10 patients who complained of attacks of abdominal pain after food (16.1 to 29.8%, p<0.01; 6.8 to 9.6 cm H(2)O, p<0.05; 135 to 350, p<0.05), and after the magnesium sulphate three of these patients experienced an attack of their usual pain. These findings provide further evidence that ;functional' abdominal pain after food may in some cases be related to an exaggerated intestinal motor response to cholecystokinin.