Natural motilin was investigated for its effect on gastrointestinal motor activity in anesthetized dogs. In doses starting from 10 ng/kg as single injection and 100 ng/kg·h as intravenous infusion, motilin induced phasic pressure changes in the lower esophageal sphincter (LES) and in the lumen of the fundus, antrum, and duodenum. Tachyphylaxis was seen in 6 out of 10 dogs. Motilin had no effect on isolated muscle strip preparations of the LES, fundus, antrum, and duodenum in this species.