Slow EPSPs evoked in the neurons of the rabbit isolated superior cervical ganglion were studied using intracellular microelectrodes. Two types of EPSPs occurring in different neurons were found. The type I slow EPSPs showed an increase during hyperpolarization of the membrane and a decrease during its depolarization. Input resistance of the neurons during the response either decreased or remained unchanged. The type II slow EPSPs were increased by depolarization and decreased by hyperpolarization with the reversal potential -78.9 +/- 3.6 mV. Depolarization evoked by acetylcholine or carbocholine was followed by an increase in the input resistance in 53% of neurons with reversal potential -83.2 +/- 6.7 mV. It is concluded that in the first group of the neurons the nature of the slow EPSP is similar to that of ordinary EPSP. The main component underlying the ionic mechanism of slow EPSP in the other group of the neurons is a decrease in potassium conductance of the membrane.