Fifty-two supraoptic nucleus neurons in rat slice preparations were studied using whole cell patch-clamp technique. The mean passive and active membrane properties were measured as follows: resting membrane potential, 59 +/- 8 mV; input resistance, 535 +/- 129 M omega; time constant, 32 +/- 9 ms; amplitude of the action potentials, 99 +/- 11 mV; overshoot, 37 +/- 13 mV (n = 39). Most of these neurons showed a prominent slow after-hyperpolarization potential or current in response to depolarizing pulse. In votage-clamp condition, it was found that virtually all supraoptic neurons (n = 13) were invaded by spontaneous synaptic inputs. Pharmacological experiments showed that the excitatory postsynaptic currents (EPSCs) were mediated by non-NMDA glutamate receptors, whereas inhibitory postsynaptic currents (IPSCs) by GABAA receptors.