Plasma motilin concentrations were measured in dogs following duodenal acidification and alkalinization and gastric instillation of fat. Antral and duodenal motility were recorded concurrently using intraluminal manometry. Alkalinization of the duodenum produced an increase in antral and duodenal motility and a significant rise in plasma motilin. Alkaline infusions at 5 mL/min into the duodenum initiated phase III of a migrating motor complex both in the antrum and in the duodenum. Duodenal acid infusions produced no change in plasma motilin concentrations while inhibiting antral motility and stimulating duodenal motility for the duration of the infusion. Gastric instillation of 60 g fat produced a 25% increase above basal motilin levels at 50 min after instillation. Motilin levels monitored during spontaneous migrating motor complexes showed peak motilin levels occurring during maximal activity of the antral duodenal region in seven out of nine motor complexes examined but motilin peaks also occurred without migrating complexes being present in this area and, as well, complexes occurred when motilin was undetectable. These results taken together with our other studies in man confirm that a true species difference exists between man and dog in the hormonal motor response to duodenal alkalinization. Although a relationship appears to exist between the appearance of maximal migrating motor complex activity in the gastroduodenal area and plasma motilin concentrations in dogs as in humans, the motilin peaks are probably neither necessary nor sufficient to induce phase III activity.