The most distal part of the circular muscle layer functions as the internal anal sphincter, which constitutes a high pressure zone at rest, but maintains a relaxed state during defecation. To elucidate such sphincter mechanisms of the smooth muscle cells, the circular muscle layer in the canine anal canal was examined within 2 cm from the anal verge. Both the mechanical and intracellular electrical activities were recorded simultaneously. The examined region could be divided into three different regions according to the pattern of spontaneous activity and innervation and consisted of an upper region (20-15 mm from the anal verge), a transitional region (15-5 mm from the anal verge) and a lower region (within 5 mm from the anal verge), respectively. The spontaneous membrane activity was characterized by ongoing slow potential changes and each potential change was associated with a phasic contraction in the three regions. The mean frequencies of spontaneous electrical activity were 6.8, 15.9, and 24.1 c/min in the upper, transitional and lower regions, respectively. In the transitional and lower region, muscle tone generation was observed. Transmural field stimulation (0.4 msec in pulse duration) evoked membrane depolarization and contractions in the lower region. The application of an alfa adrenergic blocking agent completely suppressed the generation of excitatory responses, leaving a long lasting hyperpolarization associated with relaxation. In the transitional and upper region, stimulation consistently evoked membrane hyperpolarization with relaxation. The characteristics of this hyperpolarization response varied among the three regions. The total duration of hyperpolarization increased distally, while the time to peak hyperpolarization became decreases in a reverse direction. These regional differences in the characteristics of spontaneous membrane activity and innervation indicate that the transitional and lower region might therefore function as the internal anal sphincter.