Nifekalant (NF) is a novel class III antiarrhythmic agent that is effective in preventing life-threatening ventricular tachycardia/fibrillation (VT/VF). We investigated mechanisms of destabilization and early termination of spiral-type reentrant VT by NF in a 2-dimensional subepicardial myocardial layer of Langendorff-perfused rabbit hearts (n=21) by using a high-resolution optical action potential mapping system. During basic stimulation, NF (0.1 µM) caused uniform prolongation of action potential duration (APD) without affecting conduction velocity, and an increase of APD restitution slope. VTs induced by DC stimulation in the presence of NF were of shorter duration (VTs>30s: 2/54, NF. vs. 19/93, control). During VTs in control (with visible rotors), the wavefront chased its own tail with a certain distance (repolarized zone), and they seldom met each other. Average number of phase singularity points (PSs) was 1.31±0.14/665 ms (n=7). In the presence of NF, the wavefront frequently encountered with its own tail, causing a transient breakup of the spiral-wave or sudden movement of the rotation center (spatial jump of PS). Average number of PSs was increased to 1.63±0.22/665 ms (n=7, P<0.05) after NF. The mode of spontaneous termination of rotors in control was in most cases (9/10, 90.0%) the result of mutual annihilation of counter-rotating wavefronts. With NF, rotors frequently terminated by wavefront collision with the atrioventricular groove (12/19, 63.2%) or by trapping the spiral tip in a refractory zone (7/19, 36.8%). Destabilization and early termination of spiral-wave reentry induced by NF are the result of limited proportion of excitable tissue after modulation of repolarization.