Electrical slow waves were recorded by intracellular electrodes and by quasi-intracellular pressure and suction electrodes from muscle fibers at different levels in edgewise preparations of cat jejunum. Simultaneous recordings from longitudinal and circular muscle layers showed similar resting potentials from either muscle layer near the boundary zone, and lower resting potentials in cells of circular muscle near the submucosa. Slow waves were maximal in amplitude at the boundary between the two layers and spread electrotonically away from the boundary in both layers. Bipolar recordings were of opposite polarity on the two sides of the boundary. Amplitudes of slow waves from inner circular fibers were significantly lower than from outer circular fibers. Small strips of each muscle layer were prepared with or without the attached interstitial cells of Cajal plexus as identified by methylene blue staining. Either muscle layer showed slow waves from regions where interstitial cells of Cajal (ICC) were observed after the recording. No slow waves were recorded from either layer from regions where ICC were not observed. Strips containing ICC but not strips lacking ICC could be driven electrically. Since blocking of neurons does not abolish slow waves and since regions of muscle lacking ICC do not have slow waves, it is concluded that the interstitial cells (ICC-I) are most likely the boundary elements essential for slow waves in either layer of intestinal muscle.

SUMMARY1. Smooth muscle from cat small intestine shows two types of spontaneous slow electrical waves in the frequency range of 10-15 min-'. One type of slow wave is a ouabain-sensitive, atropine-insensitive spontaneous oscillation. The other type of wave can be induced by acetylcholine (ACh), is blocked by atropine, and is not blocked by ouabain.2. Ouabain-sensitive slow waves rise directly from the baseline, are near sinusoidal and may or may not have spikes. ACh-induced waves ha' e pre-potentials, are usually topped by spikes and show after-hyperpolarization. 3. The two types of rhythmic wave differ in ionic and metabolic requirements and drug sensitivity. Ouabain-sensitive waves occur only in intestinal muscle attached to a boundary layer containing interstitial cells; ACh-induced waves can occur in strips of muscle lacking boundary cells.4. Na+ pump inhibitors ouabain, cold and K+-free solution, reduce amplitude but not frequency of ouabain-sensitive slow waves.5. The ACh-induced waves require higher extracellular concentrations of Na+ and Ca2+ and can occur in preparations in Li+-Krebs solution; the ouabain-sensitive rhythm persists in lower concentrations of Na+ and Ca2+ and is not supported by Li+. The ouabain-sensitive waves are more sensitive to cyanide and less sensitive to cooling than the ACh-induced waves.6. Guinea-pig intestine shows only one type of rhythmic wave, which is atropine sensitive and resembles in shape the ACh-induced wave of other species. Ouabain increases the frequency of the guinea-pig rhythm.7. It is concluded that intestinal muscle of most mammals, but not of guinea-pig, is capable of two types of slow electrical rhythms.