In the canine proximal colon, tissue near the submucosal surface of the circular muscle layer produces spontaneous mechanical contractions, synchronized with electrical slow waves. Comparative physiological examination of tissue strips from various regions of the submucosa and circular muscle coat revealed that the characteristic smooth muscle tissue of the innermost sublayer of the circular muscle is required for this rhythmical phenomenon. Histological examination showed that tissues containing special smooth muscle cells form an inner sublayer of the circular muscle coat. These innermost muscle cells were distinguishable from the bulk circular muscle cells by the following features: 1) flattened and shorter shapes of the cell and nucleus, 2) numerous caveolae on the cell surface, 3) abundant mitochondria, and 4) frequent gap-junction formations. Neither slow waves nor spontaneous mechanical rhythmicities were recorded from the submucosal connective tissue or from the bulk circular muscle tissue without the inner sublayer. The thicker smooth muscle cells found in the submucosal border specimens were identical in histological features to the bulk circular muscles which produced no slow waves and no spontaneous contractions. Cellular elements in the interstitium, such as fibroblasts, mast cells and macrophages, were found in all tissue strips that were physiologically examined. Nerve elements were found in all the specimens; however, there was a unique nerve network probably corresponding to the plexus entericus (submucosus) extremus described by STACH (1972). It was concluded, therefore, that the inner sublayer characterized by special smooth muscle cells with a delicate nerve plexus is essential for producing spontaneous activities of the circular muscle coat in the canine proximal colon.