The effects of neurotensin on colonic motility were investigated in 6 healthy volunteers (rectosigmoid area) and 7 patients (ascending colon and at the splenic flexure). Neurotensin (12 pmol/kg X min) infused intravenously for 30 min increased the duration of the contractions to 76% in the ascending colon and 46% in the rectosigmoid area. In the postinfusion period, the values were 42% and 67%, respectively. The motor activity did not change significantly at the splenic flexure. During the infusion period, the motility index increased from 870 to 4500 in the ascending colon and from 332 to 1700 in the rectosigmoid area. In the rectosigmoid area, however, a statistically significant increase was recorded first after cessation of the infusion. All subjects reported increased sensation of intestinal movement after intravenous infusion of neurotensin, and the patients discharged a median volume of 600 ml of bowel contents 20 min after cessation of the infusion. The data show that neurotensin causes an increase in colonic motility in the ascending colon of patients and also, after a latent period, in the rectosigmoid area of healthy subjects.