The colonic motor correlates of defecation were studied in 5 conscious dogs. A set of six strain-gauge transducers were implanted on the colon of each dog. An implanted cannula gave access to the terminal ileum. During a total control recording period of 230 h we observed 12 large-amplitude contractions that occurred spontaneously in the proximal colon and migrated caudad. We called them giant migrating contractions. The mean amplitude of these contractions was 2.8 times larger than the mean peak amplitude of phasic contractions during colonic motor complexes. The following stimuli were applied to induce defecation: 2 mg/kg guanethidine (i.v.), 30 micrograms/kg neostigmine (i.v.), 1-4 ml/kg castor oil (p.o.), 200 ml of 25% glucose (into ileum), and rectal distention by a balloon (120 ml). In 85% of experiments with guanethidine, neostigmine, glucose, and castor oil, giant migrating contractions occurred before defecation. The giant migrating contractions migrated over the entire colon or a part of its length. The migration velocity varied from 0.2 to 3.2 cm/s (mean +/- SE, 0.82 +/- 0.1 cm/s). In 11% of the experiments, giant contractions occurred almost simultaneously at different recording sites at the time of defecation. In 4% of the experiments giant contractions occurred only at a single site. Balloon expulsion was only rarely accompanied by giant contractions in the colon, and then occurred only at a distal site and did not migrate. We conclude that the colon has spontaneous but infrequent large-amplitude caudad-migrating contractions. These contractions may be the motor equivalent of mass movements. Defecation is usually preceded by colonic giant migrating contractions. The giant migrating contractions may provide a major force for defecation and be partially responsible for the evacuation of the colon during defecation. However, evacuation of contents such as a balloon seems to be possible without giant migrating contractions.