Many laboratories are pursuing the use of gene and cell therapies to treat bradyarrhythmias, an approach that has been termed creating a biological pacemaker. While divergent approaches have been explored, recent efforts have focused on exogenous expression of genes from the HCN family, or on the use of cells that endogenously express members of this gene family. The HCN gene family is responsible for the pacemaker current, I_f, which contributes to normal automaticity in the sinoatrial node. The members of this family differ in their gating characteristics and cAMP responsiveness. Thus, one question being asked is which HCN gene is optimal for creating a biological pacemaker, or whether a mutated form of HCN would function better than any of the individual native isoforms. To fully assess functionality of any biological pacemaker, it must be studied in an in vivo setting where the normal pacemaking activity is suppressed. To do this safely requires the simultaneous implantation of an electronic pacemaker in a tandem configuration, where the electronic device operates as a demand pacemaker set at a low rate. This approach has several advantages: 1) it provides an increased level of safety, and mirrors what is the likely configuration of any initial clinical trial; 2) the electronic pacemaker provides a continuous record of performance of both the biological and electronic pacemakers. We report results with an HCN2 based biological pacemaker studied in tandem mode and delivered both as a gene therapy delivered in an adenoviral vector and as a cell therapy delivered via an adult human mesenchymal stem cell. In both cases, the electronic pacemaker accounts for less than 1/3 of the heartbeats under optimal implantation conditions. In both forms of therapy, the HCN2 based biological pacemaker is able to maintain an average heart rate on the order of 55 beats per minute. The tandem pacemaker also exhibited greater responsiveness to autonomic agonists than is observed when the electronic pacemaker is operating alone. We also have investigated whether variations on the HCN2 gene result in better performance when studied in a tandem configuration. We studied a point mutation in the murine HCN2 gene (E324A) which results in a current that activates more quickly and at less negative voltages. However, these benefits were opposed by poorer expression of the exogenous gene. We also studied a chimeric channel, in which the transmembrane portion of HCN2 has been replaced by HCN1. This results in a current which activates faster than HCN2 but with similar voltage dependence when expressed in myocytes using a gene therapy approach. Unlike HCN2 based gene therapy, the chimeric channel was associated with periods of ventricular tachycardia in vivo. In summary, an HCN2 based biological pacemaker operating in tandem with an electronic pacemaker provides a level of performance in terms of basal heart rate, autonomic responsiveness, and reduced battery drain that is potentially superior to that of an electronic pacemaker alone. It remains to be determined if variations on the native HCN genes can provide improved functionality than HCN2 in this configuration.