The hypothesis was tested that interstitial cells of Cajal can generate slow wave activity. Intracellular recordings were performed only in the most superficial cells at the submucosal surface of the canine colonic circular muscle layer. An omnipresent and characteristic slow wave activity was present in all cells with a mean amplitude of 37 +/- 3 mV, a frequency of 4.6 +/- 0.1 counts/min (cpm), and a duration of 5.6 +/- 0.5 s; the average resting membrane potential was -70 +/- 1 mV. To determine the type of cell from which these recordings were obtained, methylene blue was injected by microiontophoresis. The strips were immediately fixed while the microelectrode was kept in the cell. A small segment of the tissue containing this cell was then processed for electron microscopy and serially sectioned. Electron-microscopic evidence showed that the microelectrode tip was positioned in an interstitial cell of Cajal (ICC): 1) several sections were observed with round cytoplasmic lesions of decreasing diameter followed by sections from the same cell without the lesion and 2) electron-dense material was observed in these sections due to the injected methylene blue. These cells were identified as part of the ICC network present at the muscle-submucosa interface of the circular muscle and were positively identified as ICC by the presence of cell processes. This is the first report giving direct evidence for the occurrence of electrical slow waves in ICC. It is essential support for the hypothesis that ICC are the actual pacemaker cells of the gut musculature.