Ivabradine is a 'heart rate-reducing' agent able to slow heart rate, without complicating side-effects. Its action results from a selective and specific block of pacemaker f-channels of the cardiac sinoatrial node (SAN). Investigation has shown that block by ivabradine requires open f-channels, is use dependent, and is affected by the direction of current flow. The constitutive elements of native pacemaker channels are the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, of which four isoforms (HCN1-4) are known; in rabbit SAN tissue HCN4 is expressed strongly, and HCN1 weakly. In this study we have investigated the blocking action of ivabradine on mouse (m) HCN1 and human (h) HCN4 channels heterologously expressed in HEK 293 cells. Ivabradine blocked both channels in a dose-dependent way with half-block concentrations of 0.94 microm for mHCN1 and 2.0 microm for hHCN4. Properties of block changed substantially for the two channels. Block of hHCN4 required open channels, was strengthened by depolarization and was relieved by hyperpolarization. Block of mHCN1 did not occur, nor was it relieved, when channels were in the open state during hyperpolarization; block required channels to be either closed, or in a transitional state between open and closed configurations. The dependence of block upon current flow was limited for hHCN4, and not significant for mHCN1 channels. In summary our results indicate that ivabradine is an 'open-channel' blocker of hHCN4, and a 'closed-channel' blocker of mHCN1 channels. The mode of action of ivabradine on the two channels is discussed by implementing a simplified version of a previously developed model of f-channel kinetics.