The effects of membrane potential on the waveforms and propagation of slow waves were tested using circular muscles of the canine colon. Studies were conducted with intracellular recording techniques on cross-sectional strips of canine proximal colon. Circular muscle cells near the submucosa generated slow waves that decayed in amplitude as they spread through the circular layer. The membrane potentials of cells were less negative as a function of distance from the submucosal border. Cells near the submucosa were depolarized with elevated external K+ and electrical pulses using the partitioned chamber technique. The waveforms of depolarized submucosal cells were compared with events recorded from cells in the bulk of the circular layer. The waveform changes caused by experimental depolarization were different from the changes in waveform that occur during propagation, suggesting the latter are due to a different mechanism than depolarization. The effects of the membrane potential on syncytial input resistance and length constant were also evaluated. The results of these studies are consistent with the hypothesis that slow-wave propagation across the circular layer in canine proximal colon occurs passively.