The changes in motor activity of the colon during acute colitis were investigated in six conscious dogs. The motor activity was recorded with seven strain-gauge transducers. Colitis was induced in the entire colon by luminal perfusion of acetic acid. The dogs exhibited urgency and diarrhea with mucus and blood during colitis. The mucosa was diffusely erythematous and friable and there were scattered ulcerations over the mucosal surface. The motor activity of the colon changed in several ways during colitis: (a) the total duration per hour and the mean duration of contractile states decreased significantly; (b) the cycle length of colonic migrating motor complexes was significantly prolonged, and the nonmigrating motor complexes were almost completely absent; and (d) the incidence of giant migrating contractions increased significantly. About half of the giant migrating contractions were followed by defecation. The remaining expelled mucus or gas. Sometimes, a migrating motor complex in the colon was also followed by defecation; this was never observed in the normal state. The motor activity of the colon was still decreased and the cycle length prolonged 21 days after induction of colitis. However, the dogs were asymptomatic at this time and the mucosa looked normal at colonoscopy. The incidence of giant migrating contractions was also normal at this time. It was concluded that the dog is a good model for the study of colitis because of the similarity of symptoms with human ulcerative colitis. The phasic contractions of the colon decreases during colitis but the incidence of giant migrating contractions is increased. The diarrhea in colitis may primarily be due to the large number of giant migrating contractions in the middle and the distal colon.