Current-clamp studies of cultured leech Retzius cells revealed inward rectification in the form of slow voltage sags in response to membrane hyperpolarization. Sag responses were eliminated in Na(+)-free saline and blocked by Cs+, but not Ba2+. Voltage clamp experiments revealed a Cs(+)-sensitive inward current activated by hyperpolarization negative to -70 mV. Cs+ decreased the frequency of spontaneous impulses in Retzius cells of intact ganglia. Plateau potentials were evoked in Retzius cells following block of Ca2+ influx with Ni2+ and suppression of K+ currents with internal tetraethylammonium. Plateau potentials continued to be expressed with Li+ as the charge carrier, but were eliminated when Na+ was replaced with N-methyl-D-glucamine. A persistent Na+ current with similar pharmacology that activated positive to -40 mV and reached its peak amplitude near -5 mV was identified in voltage-clamp experiments. Inactivation of the persistent Na+ current was slow and incomplete. The current was revealed by slow voltage ramps and persisted for the duration of 5-s voltage steps. Persistent Na+ current may underlie Na(+)-dependent bursting recorded in neurons of intact ganglia exposed to Ca2(+)-channel blockers.