Hyperpolarization-activated currents (I_h) are present in several neurons of the central and peripheral nervous system. However, I_h in neurons of the vomeronasal organ (VNO) is not well characterized. We investigated the properties of I_h in sensory neurons from acute slices of mouse VNO. In voltage-clamp studies, I_hwas identified by the: characteristic kinetics of activation, voltage-dependence, and blockage by Cs⁺ or ZD-7288, two blockers of the I_h. Forskolin, an activator of adenylyl cyclase, shifted the activation curve for I_h to less negative potentials. A comparison of I_h properties in VNO neurons with those of heterologously expressed hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, together with RT-PCR experiments in VNO, indicate that I_h is due to HCN2 and/or HCN4 subunits. In current-clamp recordings, blocking I_h with ZD-7288 induced a hyperpolarization of 5.1 mV, an increase in input resistance, a decrease in the sensitivity to elicit action potentials in response to small current injections, and did not modify the frequency of action potentials elicited by a large current injection. It has been shown that in VNO neurons some pheromones induce a decrease in cAMP concentration, but the physiological role of cAMP is unknown. After application of blockers of adenylyl cyclase we measured a hyperpolarization of 5.1 mV in eleven of fourteen neurons, suggesting that basal levels of cAMP could modulate the resting potential. In conclusion, these results demonstrate that mouse VNO neurons express HCN2 and/or HCN4 subunits and that I_h contributes to setting the resting membrane potential and to increase excitability at stimulus threshold.