Strongly inwardly rectifying K+ (Kir2) channels are endogenously expressed in rat brains and have recently been used as a tool to reduce the neuronal activity. But little is known about the role of Kir2 channels and the chronic effect of the reduced activity on the intrinsic excitability of neurons. Here we constructed a lentiviral vector that coexpressed Kir2.1 and GFP (LvKir2.1) and infected the vector to the hippocampal slice cultures. The LvKir2.1-infected CA1 neurons showed clear inwardly rectifying K+ currents for more than 15 days. The resting membrane potential was more negative by approximately 10 mV than those uninfected or infected with the lentiviral vector expressing GFP alone. The infection of LvKir2.1 reduced the voltage change in response to current injections and the amplitude of mEPSPs with a shunting effect. The LvKir2.1 infection significantly reduced the firings evoked by depolarizing currents in the CA1 neurons. The reduction of the firing was attributed to the hyperpolarized potential rather than to the shunting effect. These reductions were limited to modest current injections, suggesting that the overexpressed Kir2.1 plays the role of a noise-filter. Moreover, the chronic overexpression of Kir2.1 downregulated the expression of the delayed rectifier potassium current in a homeostatic manner, indicating a usefulness of this viral vector to study the activity-dependent neuronal development.