In addition to severe damage to the intestinal mucosa, there is evidence based on altered transit that irradiation affects intestinal motor function. A single dose of 938 cGy to the intestine of dogs consistently produced an acute intestinal radiation syndrome consisting of vomiting and diarrhea but was not lethal. In the fasting state, the migrating myoelectric complex was uniformly interrupted. After a meal, jejunal myoelectric activity analyzed by a computer program showed a progressive decline in the number, duration, and length of migration of spike bursts. There were occasionally bizarre motility patterns consisting of clusters of migrating spike bursts. Slow waves demonstrated irregular rhythm and nonuniform morphology. They occasionally migrated in an orad direction and at times were totally uncoupled. At 24 hr and four days after irradiation, the muscle and the neural plexus were nearly normal by light microscopy, but the mucosa exhibited severe necrosis. Therefore, irradiation produces profound functional abnormalities in intestinal muscle even though the morphology is minimally altered.