The objectives of this work were to determine the origin of slow wave activity in the canine colon, to examine the slow wave characteristics in the circular and longitudinal muscle layers, and to examine the roles played by each of these layers in the generation of this activity. Extracellular electrical activity was recorded in vitro from strips of intact muscle wall and from isolated circular and longitudinal muscle using either multiple electrodes applied to one side of the specimen or two electrodes applied simultaneously to opposite sides of the tissue. Intracellular electrical activity was also studied in intact muscle, in isolated circular muscle, and in isolated longitudinal muscle. Slow waves were recorded extracellularly from circular and longitudinal muscle when the two layers formed the intact muscle wall; they were also recorded from isolated circular muscle, but not from isolated longitudinal muscle. Removal of the submucosa from the circular muscle surface abolished slow waves recorded from both isolated circular muscle and the intact specimen. Exposure of the mucosal surface of isolated circular muscle to a hypertonic solution of KCl abolished slow waves, while exposure of the serosal surface to the same concentrations had no effect. Slow waves are not generated in longitudinal muscle. Slow waves in circular muscle are dependent on the integrity of the junction between the submucosa and the innermost circular layer.