Background: High calcium(Ca) concentration can increase vulnerability of fatal ventricular arrhythmia by inducing triggered activity or automaticity. However, the role of Ca concentration in ventricular fibrillation(VF) remain unclear. The aim of this study is to investigate the effect of Ca concentration within physiologic range on wavelet characteristics and maintenance of VF. Methods: The rabbit hearts(n=3) were retrogradely perfused and action potentials from the left ventricle free wall were recorded using di-4-ANEPPS with a high speed CMOS camera (100x100 pixels, 1000 f/s). Epicardial optical action potentials were recorded with a photodiode array (26��26 mm). The programmed stimulation was applied to assess VF characteristics in two different Ca solution within physiologic range(1.25 and 2mM). Results: The action potential duration(APD) was shortened in high Ca solution(2mmol/L), compared to lower Ca solution(1.25mmol/L)(159.7 +/- 0.4 vs. 123.9 +/- 4.4 at 180ms CL). Conduction velocity(CV) has no significant difference between two solutions(1.1 +/- 0.1vs 1.0 +/- 0.06 m/s at 180ms). APD restitution curve was steeper in high Ca solution. VF was more easily developed by programmed electrical stimulation and sustained (8.7 +/-10 vs.190 +/-20sec) in high Ca solution. Dominant frequency in VF was not different between 2 solutions (15.4 vs 16.1 Hz). However, APD60 was shortened(38 +/-1vs. 67 +/-2) and diastolic interval increased at 3 min of VF(41 +/10 vs. 28 +/-3 ms), but CV was not different in high Ca solution, suggesting more focal activity as underlying mechanism. Conclusion: This study shows that high Ca concentration can sustain VF by an increase of triggered activity or automaticity, even in physiologic range. It suggests that Ca solution should be carefully administrated in ventricular fibrillation.