Well-tolerated internal atrial defibrillation shocks must be below the pain threshold, which has been estimated to be less than 1 Joule. Defibrillation of the atria with low energy is made possible by delivering shocks at the low end of the defibrillation dose-response curve. We studied low-energy defibrillation in sheep to test the hypothesis that the energy that defibrillates the atria 10% of the time (ED10) is less than 1 Joule. The ED10 was estimated in seven sheep with rapid pacing induced chronic atrial fibrillation (AF). Low-energy defibrillation shocks were delivered from coronary sinus (CS) to superior vena cava (SVC) and the ED10 and ED50 (energy that defibrillates the atria 50% of the time) were then calculated using logistic regression. The mean ratio of ED10 to ED50 was 0.50, indicating that on average, the ED10 was equal to half of the ED50. ED10 shocks had energies ranging from 1.2 to 5.8 Joules. These results suggest that painless single-shock low-energy defibrillation may not be feasible.