The hyperpolarization-activated current I_f that is generated by the class of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels plays an important role in the control of the autonomous heart beat. There is evidence that the HCN4 isoform represents the major HCN type expressed in primary cardiac pacemaker cells of the sinoatrial node (SAN) region. Accordingly, mutations in the HCN4 gene have been identified in patients suffering from sinus bradycardia. By analyzing a knockout mouse model, we now show that HCN1, an HCN channel type whose function was so far only examined in neurons is also implicated in the control of cardiac pacemaker activity. Based on the characterization of pacemaker mechanisms in single isolated node cells, explanted beating SAN preparations and telemetric in vivo electrocardiography (ECG) we propose that HCN1 stabilizes the leading pacemaker region within the SAN giving rise to stable heart beat. Lack of HCN1 results in bradycardia, sinus dysrhythmia, recurrent sinus pauses and chronotropic incompetence. Furthermore, we show that HCN channel function is not restricted to cardiac pacemaking but is also important for normal ventricular function. In particular, we find that HCN channels confer a depolarizing background current that regulates the ventricular resting potential and counteracts the action of hyperpolarizing potassium current in late repolarization. In agreement with this role, mice deficient for HCN3 display a profound increase in the T-wave amplitude in the ECG. In conclusion, cardiac HCN channels are involved in a broader range of functions than originally assumed. This functional diversity must be taken into account when developing drugs targeting HCN channels.