Two types of slow depolarization were recorded in AH-type guinea pig myenteric plexus neurons when the myenteric plexus-longitudinal muscle preparation was stimulated transmurally with external electrodes. One depolarization was associated with a fall and the other with a rise in membrane resistance, the latter type (slow EPSP) being encountered about six times more commonly than the former. In some instances both types of potential were recorded in the same AH neuron. When this occurred the amplitude and duration of the slow EPSP was attenuated if it was timed to occur at about the same time as the other slow synaptic potential.