Hyperpolarization-activated inward currents ( I_h) contribute to neuronal excitability in sensory neurons. Four subtypes of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels generate I_h, with different activation kinetics and cAMP sensitivities. The aim of the present study was to examine the postnatal development of I_h and HCN channel subunits in trigeminal ganglion (TG) neurons. I_h was investigated in acutely dissociated TG neurons from rats aged between postnatal day (P)1 and P35 with whole cell patch-clamp electrophysiology. In voltage-clamp studies, I_h was activated by a series of hyperpolarizing voltage steps from −40 mV to −120 mV in −10-mV increments. Tail currents from a common voltage step (−100 mV) were used to determine I_h voltage dependence. I_h activation was faster in older rats and occurred at more depolarized potentials; the half-maximal activation voltage ( V_1/2) changed from −89.4 mV (P1) to −81.6 mV (P35). In current-clamp studies, blocking I_h with ZD7288 caused membrane hyperpolarization and increases in action potential half-duration at all postnatal ages examined. ZD7288 also reduced the action potential firing frequency in multiple-firing neurons. Western blot analysis of the TG detected immunoreactive bands corresponding to all HCN subtypes. HCN1 and HCN2 band density increased with postnatal age, whereas the low-intensity HCN3 and moderate-intensity HCN4 bands were not changed. This study suggests that functional I_h are activated in rat trigeminal sensory neurons from P1 during postnatal development, have an increasing role with age, and modify neuronal excitability.