Background: The core of wavelets in ventricular fibrillation as wavebreak site or firing focus may play important role in maintenance of VF through sustained wavelet formation. We hypothesized that the core of spiral wave might be due to firing triggered activity. Methods: The rabbit hearts(n=3) were retrogradely perfused and action potentials and calcium transient from the left ventricle free wall were recorded using voltage and calcium dye RH237 and Rhod-2 with a high speed CMOS camera (100x100 pixels, 1000 f/s). Epicardial optical action potentials were recorded with a photodiode array (21��21 mm). The burst pacing was performed to induce VF. Results: During long duration VF of more than 15 min, the maximal dominant frequency(DF) was located on the specific site of LV anterior wall(26-32Hz). We measured conduction velocity(CV), action potential duration(APD)60, diastolic interval(DI), and calcium transient in the maximal DF site. APD shortened(47.1 +/-0.7 vs. 61.8 +/-25.7ms), DI increased( 26.7 +/-15.7 vs. 17.4 +/-11.2ms), and more frequent calcium sink holes were observed in the maximal DF site in comparison to those of other peripheral zone, suggesting formation of new spiral wave by triggered activity. Wave propagations were slow (0.05 +/-0.01 vs. 0.18 +/- 0.05 m/s) in the maximal DF site, presenting relatively small sized wavelets. The most of the core in the maximal DF site had more short APD compared to that of the periphery(APD60 in the core: APD60 in the periphery, 1:1.8-2.4) Conclusions: This study suggests that the core of spiral wave in maximal dominant frequency site could be generated by triggered activity or automaticity rather than wavebreak and may be potential therapeutic target.