Intracellular recordings obtained from thirty-nine supraoptic nucleus neurones in perfused hypothalamic explants displayed a mean resting membrane potential of -69 mV and spike amplitude of 79 mV. Voltage-current plots were linear in the hyperpolarizing direction and revealed a mean slope resistance of 197 M omega. After Na+ channel blockade with tetrodotoxin (TTX; 0.3-16 microM), the voltage-current relationship did not change significantly for hyperpolarizing pulses. An increase in spike threshold in TTX permitted visualization of a reduction in slope resistance to depolarizing current pulses. This rectification was reduced by the addition of the Ca2+ channel blockers Cd2+, Co2+ or Mn2+. High threshold TTX-resistant spikes with amplitudes ranging between 25 and 64 mV were evoked in an all-or-none manner by brief intracellular current pulses applied above 1.0 Hz. Current pulses presented at lower frequencies (less than or equal to 0.5 Hz) evoked graded responses. In seventeen of nineteen cells, prolonged depolarizing current pulses elicited repetitive firing of TTX-resistant spikes with a progressive increase in their amplitude, rise and fall times and after-hyperpolarizations. TTX-resistant spikes were reversibly abolished when CaCl2 was replaced by equimolar amounts of EGTA or by the addition of either Cd2+, Co2+ or Mn2+ to the perfusion medium. In artificial medium containing EGTA, both the shoulder on the repolarization phase of the spike and the subsequent after-hyperpolarization were reversibly abolished. Tetraethylammonium (TEA; 2-5 mM) induced prolongation of mean action potential durations from 1.9 to 12.3 ms (nineteen cells); in TTX, TEA also prolonged the duration and increased the over-all peak amplitude of the TTX-resistant (Ca2+) spike. While TEA also enhanced the amplitude of the Na+ spike, no comparable prolongation in spike duration was observed. These data indicate that somatic action potentials of supraoptic nucleus cells arise from the co-activation of a low threshold Na+ conductance and a higher threshold Ca2+ conductance; the latter is expressed as a shoulder on the repolarization phase of the action potential.