The hyperpolarization-activated cyclic nucleotide-gated (HCN) channels belong to the superfamily of pore-loop cation channels. In mammals, the HCN channel family comprises 4 members (HCN1-4) that are expressed in heart and nervous system. HCN channels are activated by membrane hyperpolarization, are permeable to Na+ and K+, and are constitutively open at voltages near the resting membrane potential. In many cases, activation is facilitated by direct interaction with cyclic nucleotides, particularly cyclic adenosine monophosphate (cAMP). The cation current through HCN channels is known as I(h); opening of HCN channels elicits membrane depolarization toward threshold for action potential generation, and reduces membrane resistance and thus the magnitude of excitatory and inhibitory postsynaptic potentials. HCN channels have a major role in controlling neuronal excitability, dendritic integration of synaptic potentials, synaptic transmission, and rhythmic oscillatory activity in individual neurons and neuronal networks. These channels participate in mechanisms of synaptic plasticity and memory, thalamocortical rhythms, and somatic sensation. Experimental evidence indicates that HCN channels may also contribute to mechanisms of epilepsy and pain. The physiologic functions of HCN channels and their implications for neurologic disorders have been recently reviewed.(1-10).