Hippocampal mossy fiber long-term potentiation (LTP) is expressed presynaptically, but the exact mechanisms remain unknown. Here, we demonstrate the involvement of the hyperpolarization-activated cation channel ( I _h ) in the expression of mossy fiber LTP. Established LTP was blocked and reversed by I _h channel antagonists. Whole-cell recording from granule cells revealed that repetitive stimulation causes a calcium- and I _h -dependent long-lasting depolarization mediated by protein kinase A. Depolarization at the terminals would be expected to enhance transmitter release, whereas somatic depolarization would enhance the responsiveness of granule cells to afferent input. Thus, I _h channels play an important role in the long-lasting control of transmitter release and neuronal excitability.