Magnocellular neurosecretory cells (MNCs) were isolated from the supraoptic nucleus of rat hypothalamus, and properties of K(+) channels that may regulate the resting membrane potential and the excitability of MNCs were studied. MNCs showed large transient outward currents, typical of vasopressin- and oxytocin-releasing neurons. K(+) channels in MNCs were identified by recording K(+) channels that were open at rest in cell-attached and inside-out patches in symmetrical 150 mM KCl. Eight different K(+) channels were identified and could be distinguished unambiguously by their single-channel kinetics and voltage-dependent rectification. Two K(+) channels could be considered functional correlates of TASK-1 and TASK-3, as judged by their single-channel kinetics and high sensitivity to pH(o). Three K(+) channels showed properties similar to TREK-type tandem-pore K(+) channels (TREK-1, TREK-2 and a novel TREK), as judged by their activation by membrane stretch, intracellular acidosis and arachidonic acid. One K(+) channel was activated by application of pressure, arachidonic acid and alkaline pH(i), and showed single-channel kinetics indistinguishable from those of TRAAK. One K(+) channel showed strong inward rectification and single-channel conductance similar to those of a classical inward rectifier, IRK3. Finally, a K(+) channel whose cloned counterpart has not yet been identified was highly sensitive to extracellular pH near the physiological range similar to those of TASK channels, and was the most active among all K(+) channels. Our results show that in MNCs at rest, eight different types of K(+) channels can be found and six of them belong to the tandem-pore K(+) channel family. Various physiological and pathophysiological conditions may modulate these K(+) channels and regulate the excitability of MNCs.