Background: The ultimate acceptance of a fully automatic atrial defibrillator will depend on the reduction of pain in acceptable levels, requiring a marked decrease in defibrillation thresholds. The purpose of this study was to determine whether atrial defibrillation thresholds can be reduced by sequential shocks delivered through two current pathways. Methods and Results: Sustained atrial fibrillation was induced with rapid atrial pacing in 12 adult sheep. Defibrillation electrodes were positioned in the right atrial appendage (RAap), distal coronary sinus (DCS), proximal coronary sinus (CSos), main/left pulmonary artery junction (PA), and right ventricular apex (RV). Single-capacitor biphasic waveforms (3/1 ms) were delivered through combinations of these electrodes. Probability-of-success curves were determined for single shocks with a single current pathway and sequential shocks with either single- or dual current pathways. The ED50 for delivered energy for the dual current pathway RAap to DCS then CSos to PA was 0.36±0.13 J, which was significantly lower than the ED50 of the standard single current pathway RAap to DCS (1.31±0.3 J) and was significantly lower than all other configurations tested. Conclusions: Internal atrial defibrillation thresholds can be markedly reduced with two sequential biphasic shocks delivered over two current pathways compared with the standard single shock delivered over a single current pathway or with sequential shocks delivered over a single current pathway.