It has been reported that blockade of the inward rectifier K+ current (I-K1) facilitates termination of ventricular fibrillation. We hypothesized that partial I-K1 blockade destabilizes spiral wave (SW) re-entry, leading to its termination. Optical action potential (AP) signals were recorded from left ventricles of Langendorff-perfused rabbit hearts with endocardial cryoablation. The dynamics of SW re-entry were analyzed during ventricular tachycardia (VT), induced by cross-field stimulation. Intercellular electrical coupling in the myocardial tissue was evaluated by the space constant. In separate experiments, AP recordings were made using the microelectrode technique from right ventricular papillary muscles of rabbit hearts. Ba2+ (10-50 mu M) caused a dose-dependent prolongation of VT cycle length and facilitated termination of VT in perfused hearts. Baseline VT was maintained by a stable rotor, where an SW rotated around an I-shaped functional block line (FBL). Ba2+ at 10 mu M prolonged I-shaped FBL and phase-singularity trajectory, whereas Ba-2 at 50 mu M transformed the SW rotation dynamics from a stable linear pattern to unstable circular/ cycloidal meandering. The SW destabilization was not accompanied by SW breakup. Under constant pacing, Ba2+ caused a dose-dependent prolongation of APs, and Ba2+ at 50 mu M decreased conduction velocity. In papillary muscles, Ba2+ at 50 mu M depolarized the resting membrane potential. The space constant was increased by 50 mu M Ba2+. Partial I-K1 blockade destabilizes SW rotation dynamics through a combination of prolongation of the wave length, reduction of excitability, and enhancement of electrotonic interactions, which facilitates termination of ventricular tachyarrhythmias.