We investigated detailed extracellular photosensitization reaction effect on rat myocardial cells and the photosensitization reaction progress in a well to study a new application of photodynamic therapy for arrhythmia therapy. OCIS codes: (170.0170) Medical optics and biotechnology; (170.5180) Photodynamic therapy 1. Introduction Radio-frequency catheter ablation is widely applied for arrhythmia treatment . The coagulation necrosis induced by Joule's heat blocks the abnormal impulse propagation that causes arrhythmia. The temperature at the tissue surface may be controlled in 60 to 70° ranges but excessive temperature rise inside the tissue induces severe thermal complications [2,3]. We have proposed a new non-thermal ablation methodology for arrhythmia using extracellular photosensitization reaction to solve these problems in current arrhythmia ablation [4-6]. The laser irradiation shortly after the photosensitizer injection was employed. Talaporfin sodium (Meiji Seika Pharma Co., Ltd., Japan) that has high singlet oxygen production efficiency and quick egestion was used as the photosensitizer in our proposed method. The extracellular photosensitization reaction may induce not only permanent electrical conduction block by cell necrosis but also immediate electrical conduction block by cell membrane and/or ion channel damage . The effect of the extracellular photosensitization reaction on myocardial cells should be investigated to optimize the photosensitization reaction parameters on clinical application. The photosensitization reaction situation in experimental setup in vitro using a 96 well plate may be completely different from that of the intracellular photosensitization reaction since the number of the photosensitizer molecule in the well is 105 fold larger. This photosensitizer rich condition might induce dissolved oxygen depletion by high oxygen consumption. The myocardial cell lethality after the extracellular photosensitization reaction was investigated varying the photosensitizer concentrations and radiant exposures widely. The photosensitization reaction progress in a well of the 96-well plate that is generally used to cell study was investigated by measuring the photosensitizer fluorescence, oxygen pressure, photosensitizer solution absorbance, and solution temperature.