Background - The standard lead configuration for internal atrial defibrillation consists of a shock between electrodes in the right atrial appendage (RAA) and coronary sinus (CS). We tested the hypothesis that the atrial defibrillation threshold (ADFT) of this RAA→CS configuration would be lowered with use of an additional electrode at the atrial septum (SP). Methods and Results - Sustained atrial fibrillation was induced in 8 closed-chest sheep with burst pacing and continuous pericardial infusion of acetyl-β-methylcholine. Defibrillation electrodes were situated in the RAA, CS, pulmonary artery (PA), low right atrium (LRA), and across the SP. ADFTs of RAA→CS and 4 other lead configurations were determined in random order by use of a multiple-reversal protocol. Biphasic waveforms of 3/1-ms duration were used for all single and sequential shocks. The ADFT delivered energies for the single-shock configurations were 1.27±0.67 J for RAA→CS and 0.86±0.59 J for RAA+CS→SP; the ADFTs for the sequential-shock configurations were 0.39±0.18 J for RAA→SP/CS→SP, 1.16±0.72 J for CS→SP/RAA→SP, and 0.68±0.46 J for RAA→CS/LRA→PA. Except for CS→SP/RAA→SP versus RAA→CS and RAA→CS/LRA→PA versus RAA + CS→SP, the ADFT delivered energies of all of the configurations were significantly different from each other (P<0.05). Conclusions - The ADFT of the standard RAA→CS configuration is markedly reduced with an additional electrode at the atrial SP.