Background: Short term cardiac memory is related to electrical restitution curve correlating to vulnerability of ventricular fibrillation. Ik1 blocker, Barium(Ba) is a potent agent of VF suppression. We assessed the effect of Ik1 on short-term cardiac memory in ventricular fibrillation. Methods:The rabbit hearts(n=3) were retrogradly 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). Programmed electrical stimulation was performed to assess restitution curve after Ba administration. Results : Long duration VF (4 episode) was terminated by the administration of Ba 50關M/L. Action potential duration (APD) and conduction velocity (CV) showed markedly heterogeneous spatiotemporal alternans at every stimulation intervals(Max 100 +/- 12ms Min 60+/-13ms in APD, Max 0.5 +/-0.2 m/s, Min 0.2 +/-0.1 m/s in CV at 280ms). However, APD restitution curve was flattened compare to control, reflecting impaired cardiac memory. Paced wave-front did not propagate, producing multiple wavelets but sustained VF did not occur. Conclusions: This study suggests that inward rectifying potassium potassium current Ik1 is potent mechanism of cardiac memory and that Ik1 blocker terminates VF by inducing impairment of cardiac memory.