Hyperpolarization-activated cyclic nucleotide gated non-specific (HCN) channels are widely expressed in hippocampal neurons. Although they are involved in determining resting membrane potential, their role in synaptic function is poorly understood. We investigated the involvement of HCN channels in synaptic plasticity by using field excitatory postsynaptic potential (fEPSP) recordings at the Schaffer collateral-CA1 synapses. Application of the specific HCN channel blocker ZD7288 (10 mM) decreased paired-pulse ratio (PPR) using interstimulus intervals of 10–200 ms. We next examined the effect of ZD7288 on the induction of (RS)-3,5-dihydroxyphenylglycine-induced long-term depression (DHPG-LTD). Following a brief application of DHPG (100 mM, 10 min), CA1 fEPSPs were strongly depressed resulting in stable LTD after 60 min of wash-out (69±4% of control). In marked contrast, administration of ZD7288 significantly enhanced the DHPG-LTD (41±8% of control, p<0.05), indicating an impairment of DHPG-LTD by HCN channels. To identify the site of ZD7288 action on LTD enhancement, we calculated the PPR (interstimulus interval 40 ms) following DHPG-LTD normalized to experiments without DHPG. After washout of DHPG for 1 hr, the control PPR was 107±1% compared to 136±3% in bath solution containing ZD7288 (p<0.05). In summary, HCN channels also seem to impair DHPG-induced LTD by a presumably presynaptic mechanism.