High energy defibrillation shock is the only therapy for ventricular tachyarrhythmias. However, due to adverse side effects, lowering defibrillation energy is desirable. We investigated mechanisms of unpinning, destabilization, and termination of ventricular tachycardia (VT) by low energy shocks in isolated rabbit right ventricular preparations (n = 22). Stable VT was initiated with burst pacing and optically mapped. Monophasic "unpinning" shocks (10 ms) of different strengths were applied at various phases throughout the reentry cycle. In 8 of 22 preparations, antitachycardia pacing (ATP: 8-20 pulses, 50-105% of period, 0.8-10 mA) was also applied. Termination of reentry by ATP was achieved in only 5 out of 8 preparations. Termination by unpinning occurred in all 22 preparations. Rayleigh's test showed a statistically significant unpinning phase-window, during which reentry could be unpinned and subsequently terminated with E80 (magnitude at which 80% of reentries were unpinned) = 1.2 V/cm. All reentries were unpinned with field strengths at or below 2.4 V/cm. Unpinning was achieved by inducing VEP and secondary sources of excitation at the core of reentry. Optical mapping revealed the mechanisms of phase-dependant unpinning of reentry. These results suggest that a 20-fold reduction in energy could be achieved compared to conventional high energy defibrillation and that the unpinning method may be more effective than ATP for terminating stable, pinned reentry in this experimental model.