The effect of various nutrients and hormones on motilin release was examined in normal volunteers and postvagotomy patients to investigate the mechanism of motilin secretion in the postprandial state. The ingestion of a mixed meal, protein and fat elevated the plasma motilin level, but the oral glucose load and arginine infusion lowered it. Although gastrin infusion showed no effect on the plasma motilin concentration, the infusion of insulin or glucagon inhibited motilin release in vivo. The patients, who underwent a selective or truncal vagotomy, also revealed a rise in plasma motilin after meal ingestion, suggesting that motilin could be released even in the absence of gastric acid and vagal stimulus. The perifusion experiments demonstrated that motilin release from human duodenal mucosa into the perfusate was stimulated markedly by low pH and 15 mM taurocholate, but not affected by the perifusion of 20 mM glucose, 20 mM arginine, 100 mU/l insulin or 30 nM glucagon. These results indicate that the motilin release induced by meal ingestion depend upon the balance of food components and that in normal subjects duodenal acidification, bile acid and unknown factors contained in the mixed meal may participate at least in part in postprandial motilin release. The inhibitory effect of glucose on motilin release may be attributable to a certain mediator mechanism which remains to be studied.