In four dogs we quantified the role of external ("coloileal") ligaments in preventing coloileal reflux. All animals were tested under control conditions, and then two had all external ligamentous attachments between ileum and colon divided; the other pair underwent a sham operation. Coloileal reflux was quantified scintigraphically at colonic pressures of 20, 40, and 60 mm Hg, and ileal motility was recorded concurrently. During control experiments and after sham operations, no dogs showed coloileal reflux at colonic pressures of 20 and 40 mm Hg. At a colonic pressure of 60 mm Hg, two control experiments and one in a dog after sham operation resulted in reflux of 9%, 4%, and 8% of counts, respectively. In contrast, both test dogs (after division of the ligaments) refluxed 30-70% of colonic content in all of four experiments at pressure below 20 mm Hg. In control dogs and in those with a continent ileocolonic junction, ileal motility consisted of scattered clusters of phasic contractions. In dogs with coloileal reflux, these clusters occurred with a similar frequency, but they lasted longer (P less than 0.005). Four weeks later, ileal motility indices in control dogs were significantly less (P less than 0.02) than in animals with divided coloileal ligaments. These observations establish an experimental model for coloileal reflux, support the hypothesis that external ligamentous attachments help maintain continence at the ileocolonic junction, and imply that coloileal reflux changes the motor pattern of the terminal ileum.