We have shown that the medial preoptic area (MPO) in the hypothalamus is a major site where interferon (IFN)-α acts to induce suppression of splenic natural killer (NK) cell activity through an activation of sympathetic nervous system (SNS) in rats. Here, we discuss the hypothalamic mechanisms of the cytokine action using in vivo and in vitro preparations in rats. Lesion of the MPO activated the SNS and suppressed splenic NK cell activity in anesthetized rats, suggesting that the MPO had an inhibitory influence on nerve activity. Since both IFN-α and interleukin (IL)-1β are known to suppress MPO neuron activity, it is suggested that the suppression/loss of the MPO caused by cytokine actions/lesions disinhibits the hypothalamic-sympathetic pathway, thereby resulting in an increase in the splenic SNS and reduction of NK activity. To explore the cellular mechanisms of the suppression of MPO neurons, the effects of Prostaglandin E2 (PGE2), one of the major mediators of cytokine action in the brain, on the glutamate-induced membrane currents were examined using the perforated patch-clamp method in mechanically dissociated MPO neurons. Patch-clamp analysis revealed that PGE2 potentiated the Ca2+-dependent K+ current (KCa) stimulated by Ca2+ entry through N-methyl-D-aspartate channels. We suggest that the cytokine-induced decrease in the firing rates of MPO neurons may be a result of an increase in interspike intervals caused by PGE2-induced enhancement of KCa in the presence of glutamatergic inputs.