Understanding the basic mechanisms of excitability through the cardiac cycle is critical to both the development of new implantable cardiac stimulators and improvement of the pacing protocol. Although numerous works have examined excitability in different phases of the cardiac cycle, no systematic experimental research has been conducted to elucidate the correlation among the virtual electrode polarization pattern, stimulation mechanism, and excitability under unipolar cathodal and anodal stimulation. We used a high-resolution imaging system to study the spatial and temporal stimulation patterns in 20 Langendorff-perfused rabbit hearts. The potential-sensitive dye di-4-ANEPPS was utilized to record the electrical activity using epi-fluorescence. We delivered S1-S2 unipolar point stimuli with durations of 2-20 ms. The anodal S-I curves displayed a more complex shape in comparison with the cathodal curves. The descent from refractoriness for anodal stimulation was extremely steep, and a local minimum was clearly observed. The subsequent ascending limb had either a dome-shaped maximum or was flattened, appearing as a plateau. The cathodal S-I curves were smoother, closer to a hyperbolic shape. The transition of the stimulation mechanism from break to make coincided with the final descending phase of the S-I curves and occurred for S1-S2 intervals between 185 ± 8.4 ms and 190 ± 8.4 ms (mean ± std, N = 11) for anodal stimulation and between 178 ± 7.0 ms and 183 ± 7.0 ms (mean ± std, N = 9) for cathodal stimulation. The transition is attributed to the bidomain properties of cardiac tissue. The effective refractory period was shorter when positive stimuli were delivered (129.5 ± 13.3 ms, N = 11) than for negative stimulation (134.4 ± 5.8 ms, N = 9). Our spatial and temporal analyses of the stimulation patterns near refractoriness show always an excitation mechanism mediated by damped wave propagation after S2 termination.