Pulmonary veins contain a myocardial layer, whose electrical activity is considered to be involved in the genesis and maintenance of atrial fibrillation. We have previously shown that Anemonia sulcata toxin (ATX)-Ⅱ, a potent late Na⁺ current activator, induced a persistent electrical activity in quiescent pulmonary vein myocardium. In this study, we investigated how the intracellular ion environment is involved in the electrical abnormal excitement of pulmonary vein cardiomyocytes due to late Na⁺ current activation using fluorescence microscopy. In pulmonary vein cardiomyocytes loaded with the fluorescent Na⁺ indicator, SBFI, ATX-Ⅱ increased basal fluorescence ratio. ATX-Ⅱ also increased the basal fluorescence intensity of Fluo-4, a Ca²⁺ indicator, and induced spontaneous Ca²⁺ oscillations such as Ca²⁺ spark and Ca²⁺ transient. These phenomena were significantly suppressed by pretreatment with SEA0400, an inhibitor of the Na⁺/ Ca^{2 +} exchanger. These results indicated that the ATX-Ⅱ-induced increase in intracellular Na⁺ concentration activates the reverse-mode Na⁺/ Ca²⁺ exchanger and indirectly increases the intracellular Ca²⁺ concentration.