The ultramicroelectrode technique with Trenndelenburg''s method were used to study electrical activities in isolated guinea-pig jejunal loops. Increased intraluminal loop pressure, increased muscle cell membrane depolarization gradually, at 1st oscillating minutely, and then summating to develop slow depolarizing waves with a longer duration. When the peak of the depolarizing wave reaches a certain critical level, i.e., about 12 mV, a spike is initiated, which is successively followed by an initial after-hyperpolarization (IAHP), the late after-depolarization (LADP) and a delayed after-hyperpolarization (DAHP), the latter soon changing the increased depolarization phase, prepotential (PP), until it culminates in the induction of the spike again. With lower pressure, the spike is abolished and the slow waves then decline in their amplitude, until they break into minute oscillations. The spike is thus responsible for fully developing the slow waves. The late after-depolarization (LADP) is highly developed by applying acetylcholine or by raising the pressure to initiate repetitive spike potentials, but are reduced after atropine or nicotine application or lowering the pressure. In the isometric condition the slow waves can be completely abolished, spikes being continually produced. The depolarizing waves ranging from minute oscillations to slow waves are essentially of the same nature. These waves are presumed to be elicited by acetylcholine released from the intramural nervous elements.