Smooth muscle cells from the circular muscle layer of the dog colon showed a mechanical threshold of -44 mV. No gradient in mechanical threshold was measured between the cells from the submucosal and myenteric plexus surface. The threshold was passed during the upstroke and the plateau phase of the spontaneous slow-wave activity from cells at the submucosal surface and by spike potentials occurring mainly in cells at the myenteric plexus surface and sporadically in cells at the submucosal surface. Carbachol-induced specific changes in electrical and mechanical activities that were inhibited by calcium influx blockade are as follows: 1) increase in slow-wave duration; 2) decrease in plateau potential; 3) enhancement of spiking activity; and 4) increase in contractility. This indicates that calcium influx is significantly increased in the presence of carbachol in cells at both surfaces of the circular muscle layer. The increase in calcium influx could be the result of a direct action by carbachol on the calcium conductance and/or could be mediated by a decrease in outward current. The latter is suggested by the carbachol-induced membrane depolarization associated with an increase in the input resistance, which were both methoxyverapamil insensitive. The results show that an excitatory stimulus can generate contraction of the circular muscle through different electrophysiological activities. In addition, the patterns of spontaneous electrical activity and the different responses to carbachol stimulation provide further information about the heterogeneous nature of the electrical activities within the colonic circular muscle layer.