In order to investigate the possible involvement of gastrointestinal hormones in functional disorders of the digestive tract, serum motilin, neurotensin and gastrin levels and their response to oral intake of fat and glucose were examined in patients with irritable colon syndrome and dumping syndrome. The following results were obtained.(1) Basal serum motilin levels were higher in patients with irritable colon syndrome than in normal subjects, and remained high after ingestion of either 50 g of butter or 50 g of glucose.(2) No consistent response in serum neurotensin levels was found in patients with irritable colon syndrome or in normal subjects.(3) An immediate increase in serum gastrin levels was found in response to fat ingestion both in patients with irritable colon syndrome and in normal subjects, but there was no difference between these two groups.(4) In a patient with typical dumping syndrome, a markedly high level of fasting serum motilin was found, and the level increased further after the oral intake of glucose. These findings suggest that motilin may be involved in the irritable colon syndrome and dumping syndrome.