Background: Atrial tachycardia (AT) and fibrillation (AF) result in rapid ventricular rates that are detrimental to optimal cardiac function. The purpose of this study is to determine if the application of a "coupled pacing" (CP) regimen would improve ventricular function by decreasing the ventricular rate of mechanical contractions (VRMC). Methods: We simulated AT by pacing either atrium at a rate that resulted in a rapid but regular ventricular rate in 7 anesthetized dogs. AF was induced by increasing the atrial-pacing rate until atrial activation did not follow the pacing. After the induction of either AT or AF, we applied CP after each intrinsic ventricular activation. We measured the VRMC, and left ventricular pressures and volumes via a pressure/conductance catheter. Results: The marked reductions in VRMCs during CP resulted in increases in left ventricular end diastolic volume. The coupled pacing resulted in virtually no mechanical contractions whereas the strength of contractions from the normal electrical activation increased. The increases in the positive dLVP/dt and left ventricular ejection fraction during CP are the result of "postextrasystolic potentiation". The average stroke work (area of the pressure volume loops) increased as the result of CP during both AT and AF. Despite the large increases in stroke volume (2x) during CP, the changes in cardiac outputs were moderate because the VRMCs markedly decreased (1/2). Conclusions: Coupled pacing therapy may be a viable therapy for slowing the heart rate and improving contractility in patients with AT and AF.