We studied cholecystokinin octapeptide (CCK-OP) for its motor effects and sites of action on human isolated muscle from stomach, small intestine and colon. 2. CCK-OP induced a concentration-dependent contraction of all the longitudinal muscles and of circular muscle from the stomach and large intestine. The peptide acted directly on these muscles at a site not involving muscarinic receptors. 3. CCK-OP relaxed the circular muscle of the small intestine and/or reduced the contractions to acetylcholine, by stimulating intramural postganglionic inhibitory neurones.