The properties and functional roles of the hyperpolarization-activated inward current (I(H)) in magnocellular neurosecretory cells (MNCs) were investigated during sharp microelectrode recordings from supraoptic neurons in superfused explants of rat hypothalamus. Under current clamp, voltage responses to hyperpolarizing current pulses featured depolarizing sags that were abolished by the I(H) blocker ZD 7288. Under voltage clamp, subtraction of current responses to hyperpolarizing steps recorded in the absence and presence of ZD 7288 was used to investigate the properties of I(H). Current-voltage analysis revealed that steady-state I(H) amplitude increases with hyperpolarization, with half-maximal activation of the underlying conductance occurring at -78 mV. The time course of activation of I(H) during hyperpolarizing steps was monoexponential with time constants (100-800 msec) decreasing with hyperpolarization. The effects of ZD 7288 on I(H) were slow (tau, approximately 15 min), irreversible, and half-maximal at 1.8 micrometer. When tested on continuously active MNCs, application of 30-60 micrometer ZD 7288 caused a significant reduction in firing rate. In phasically active MNCs, the drug decreased burst duration and intraburst firing frequency and caused an increase in the duration of interburst intervals. These effects were accompanied with a small hyperpolarization of the membrane potential. In contrast, ZD 7288 had no effect on spike duration, on the amplitude of calcium-dependent afterpotentials, or on the frequencies and amplitudes of spontaneous synaptic potentials. These results confirm the presence of I(H) in MNCs of the rat supraoptic nucleus and suggest that the presence of this conductance provides an excitatory drive that contributes to phasic and tonic firing.