The effect of intravenous infusion of gastrin II on gastric and colonic motor activity was studied in 12 patients; gastric acid output was also measured. Administration of gastrin at near-maximal dose levels stimulates the motor activity of the antrum but has no measurable effect on the activity of the proximal colon, sigmoid, or rectum. The results suggest that gastrin plays a part in the regulation of gastric motility but that it is not a mediator of the gastro-colic reflex.