Background - The atrial defibrillation threshold (ADFT) energy of the standard lead configuration, right atrial appendage (RAA) to coronary sinus (CS), was reduced by >50% with the addition of a third electrode traversing the atrial septum in a previous study. This study determined whether the ADFT would be lowered by a more clinically practical third electrode placed in the right atrium along the atrial septum (RSP). Methods and Results - Sustained atrial fibrillation was induced in 8 closed-chest sheep with burst pacing and maintained with pericardial infusion of acetyl-β-methylcholine chloride. A custom-made, dual-defibrillation catheter was placed with electrodes in the lateral RA, CS, and RSP. A separate defibrillation catheter was also placed in the RAA. ADFT characteristics of RAA→CS and 6 other single- or sequential-shock configurations were determined in random order by using biphasic, truncated-exponential waveforms in a multiple-reversal protocol. The delivered-energy, peak-voltage, and peak-current ADFTs for the sequential-shock configuration CS→RSP/RA→RSP (0.53±0.31 J, 86±22 V, and 1.6±0.6 A, respectively) were significantly lower than those of RAA→CS (1.14±0.64 J, 157±34 V, and 2.5±1.1 A, respectively). The ADFT characteristics of RAA→CS and RA→CS were not significantly different, nor were those of CS→RSP/RA→RSP and CS→RSP/RAA→RSP. Conclusions - The ADFT of the standard RAA→CS configuration may be markedly reduced with an additional electrode situated at the RSP.