By combining electrophysiological, immunohistochemical, and computer modeling techniques, we examined the effects of halothane on the standing outward current (I (SO)) and the hyperpolarization-activated current (I (h)) in rat thalamocortical relay (TC) neurons of the dorsal lateral geniculate nucleus (dLGN). Hyperpolarizing voltage steps elicited an instantaneous current component (I (i)) followed by a slower time-dependent current that represented I (h). Halothane reduced I (h) by shifting the voltage dependency of activation toward more negative potentials and by reducing the maximal conductance. Moreover, halothane augmented I (i) and I (SO). During the blockade of I (h) through Cs+, the current-voltage relationship of the halothane-sensitive current closely resembled the properties of a current through members of the TWIK-related acid-sensitive K+ (TASK) channel family (I (TASK)). Computer simulations in a single-compartment TC neuron model demonstrated that the modulation of I (h) and I (TASK) is sufficient to explain the halothane-induced hyperpolarization of the membrane potential observed in current clamp recordings. Immunohistochemical staining revealed protein expression of the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel proteins HCN1, HCN2, and HCN4. Together with the dual effect of halothane on I (h) properties, these results suggest that I (h) in TC neurons critically depends on HCN1/HCN2 heterodimers. It is concluded that the reciprocal modulation of I (h) and I (TASK) is an important mechanism of halothane action in the thalamus.