1. The potential changes were recorded from the individual large cells of the lobster cardiac ganglion with the aid of intra-cellular microelectrodes. Special attention was paid to the interaction among the cells, and, using a dual beam oscilloscope, simultaneous recording was made from two different cells.
2. The “follower” type of potential was the most common and similar to that obtained in ganglion cells of an American lobster, described by Hagiwara and Bullock (10), and Bullock and Terzuolo (7).
3. The synaptic potentials in two adjacent cells were closely synchronized. This seems to indicate existence of common presynaptic fibers. On the other hand the superimposed spikes were not always synchronized.
4. With an inward current applied to the cell membrane, the spike generation was often blocked. On the other hand, however, the synaptic potentials were increased in amplitude.
5. An outward current applied to the cell membrane caused a depolarization. With sufficient strength a spike or a series of spikes was produced superimposed on the depolarization.
6. Electrotonic potentials were elicited by an applied current, not only in the cell in which the current electrode was inserted, but also in a neighboring cell in which no current electrode was inserted, with a reduced amplitude and a delayed time course. This phenomenon was regarded as existence of “electrical connections” among large cells.
7. In some specimens when the small cells were anaesthetized with 0.1% procain-sea water, a slow oscillatory potential change was produced in one of the large cells, which has variable amplitude of up to 10 mV and frequency of about one per second, and was superimposed with a spike or a series of spikes. 8. The slow oscillatory potentials appeared in a neighboring cell with a certain phase shift and a reduced amplitude. This phenomenon was explained as a sort of electrotonic spread which is made possible by the electrical connection among cells.