1. Action potentials recorded in the soma of R15 neurones in the abdominal ganglia of Aplysia juliana were not suppressed by selective inhibition of either Na or Ca conductance alone. It was necessary to block both conductances to suppress action potentials. 2. Membrane currents generated by step depolarizations of the soma consisted of early transient and delayed steady-state currents. The early transient current could have one or two components depending on the activating depolarization. 3. The early more rapid component had a reversal potential at +54 mV and the reversal potential changed with extracellular Na concentration in accord with the Nernst equation. It was blocked by substitution of impermeant cations for Na, by TTX and by internal injections of Zn. It was concluded that this component was normally a Na current. 4. The later slower component of the transient current had a reversal potential at about +65 mV and the reversal potential changed with extracellular Ca concentration is accord with the Nernst equation. It was blocked by substitution of Mg for Ca or addition of Mn, Co, Ni or verapamil to the extracellular solution. It was concluded that this component was normally a Ca current. 5. Na and Ca currents were generated at different threshold potentials, Na currents first appearing at about -20 mV and Ca currents at -5 to 0 mV. 6. The time-to-peak of both Na and Ca currents was affected by the holding potential, by the amplitude of the activating depolarization, by temperature and by divalent ion concentration. 7. The peak Na and Ca conductances both increased sigmoidally with increasing depolarization, the maximum Na conductance of 10--15 microS being approximately twice the maximum Ca conductance. Peak conductances for Na and Ca reached half-maximum at -8 and +3 mV, respectively. 8. The amplitude of the delayed steady-state current could be varied by changing the extracellular K+ ion concentration or by adding tetraethylammonium to the extracellular solution. The reversal potential for 'tail currents' was -67 mV and shifted 18 mV when the extracellular K concentration was doubled. It was concluded that the delayed steady-state current was K current. 9. With prolonged depolarizations, K current decayed with a time constant of the order of 1 sec. Peak K conductance increased with increasing depolarization with the half-maximum occurring at a potential more positive than +20 mV. The maximum rate of fractional activation of K conductance was independent of the amplitude of the clamp step.