Objective To study the effects of long term application of cathartics on electromyography of rat colon, and to explore the role of interstitial cells of Cajal (ICC) in it. Methods Colonic slow waves of the rat was examined after 3-month feeding of phenolphthalein, and ICC in myenteric plexus was observed by ZIO method, and ultrastructure changes of nerves and ICC was observed. Results The frequency of slow waves of cathartic colon was reduced significantly(Ｐ＜0.05). The distribution of ICC in myenteric plexus was uneven, and the processes were mussily connected each other. Vacuolar degeneration of axon and ICC-like cells was revealed by electron microscope in myenteric plexus of cathartic colon. Conclusion Long term application of phenolphthalein could reduce the frequency of colonic slow waves, and the possible mechanism was degeneration of ICC and myenteric plexus nerves.