The effect of glucagon on human colonic myoelectrical activity is described. By means of intraluminal, serosal, and surface electrodes, recordings from all areas of the large bowel have been obtained. Glucagon inhibited both electrical and pressure rhythms in all subjects tested. Evidence is produced to suggest a direct action on colonic smooth muscle. A controlled trial using glucagon during routine barium enema examinations suggests that it may prove to be useful for hypotonic examinations of the colon where painful spasm is present.