The immunomodulatory cytokine interferon- (IFN-) is used in the treatment of autoimmune diseases such as multiple sclerosis. However, the effect of IFN- on neuronal functions is currently unknown. Intracellular recordings were conducted on somatosensory neurons of neocortical layers 2/3 and 5 exposed to IFN-. The excitability of neurons was increased by IFN- (10 - 10,000 U/ml) in two kinetically distinct, putatively independent manners. Firstly, IFN- reversibly influenced the subthreshold membrane response by raising the membrane resistance R_M 2.5-fold and the membrane time constant 1.7-fold dose-dependently. The effect required permanent exposure to IFN-, and was reduced in magnitude if the extracellular K⁺ was lowered. However, the membrane response to IFN- in the subthreshold range was prevented by ZD7288 (a specific blocker of I_h) but not by Ni²⁺, carbachol or bicuculline, pointing to a dependence on an intact I_h. Secondly, IFN- enhanced the rate of action potential firing. This effect was observed even after 5 min exposure to IFN- and showed no reversibility (up to 210 min). Current-discharge (F-I) curves revealed a shift (prevented by bicuculline), as well as an increase in slope (prevented by carbachol and Ni²⁺). Layer specificity was not observed with any of the described effects. In conclusion, IFN- influences the neuronal excitability in neocortical pyramidal neurons in vitro, especially under conditions of slightly increased extracellular K⁺. Our blocker experiments indicate that various ionic conductances with different voltage-dependencies may cause different influences on sub- and supra-threshold behavior, suggesting a more general intracellular process induced by IFN-.