Introduction: Little investigation has been conducted to assess the atrial defibrillation thresholds of electrode configurations using electrodes designed for internal ventricular defibrillation (right ventricle [RV], superior vena cava [SVC], and pulse generator housing [Can]) combined with coronary sinus (CS) electrodes. We hypothesized that a CS → SVC+Can electrode configuration would have a lower atrial defibrillation threshold than a standard configuration for defibrillation, RV → SVC+Can. We also tested the atrial defibrillation thresholds of five other configurations. Methods and Results: In 12 closed chest sheep, we situated a two-coil (RV, SVC) defibrillation catheter, a left-pectoral subcutaneous Can, and a CS lead. Atrial fibrillation was burst induced and maintained with continuous infusion of intrapericardial acetyl-β-methylcholine chloride. Using fixed-tilt biphasic shocks, we determined the atrial defibrillation thresholds of seven test configurations in random order according to a multiple-reversal protocol. The peak voltage and delivered energy atrial defibrillation thresholds of CS → SVC+Can (168 ± 67 V, 2.68 ± 2.40 J) were significantly lower than those of RV → SVC+Can (215 ± 88 V, 4.46 ± 3.40 J). The atrial defibrillation thresholds of the other test configurations were RV+CS → SVC+Can: 146 ± 59 V, 1.92 ± 1.45 J; RV → CS+SVC+Can: 191 ± 89 V, 3.53 ± 3.19 J; CS → SVC: 188 ± 98 V, 3.77 ± 4.14 J; SVC → CS+Can: 265 ± 145 V, 7.37 ± 9.12 J; and SVC → Can: 516 ± 209 V, 24.5 ± 15.0 J. Conclusions: The atrial defibrillation threshold of CS → SVC+Can is significantly lower than that of RV → SVC+Can. In addition, the low atrial defibrillation threshold of RV+CS → SVC+Can merits further investigation. Based on corroboration of low atrial defibrillation thresholds of CS-based configurations in humans, physicians might consider using CS leads with atrioventricular defibrillators.