We have demonstrated photosensitization reaction-induced electrical conduction block of myocardial tissue. Subcellular distribution of photosensitizer and electrophysiological cell response to this photosensitization reaction were studied with rat myocardial cells, in order to clarify the mechanism of the myocardial electrical conduction block with photodynamic therapy (PDT). Photofrin® was used as photo sensitizer in this study. The subcellular distribution of the photosensitizer was obtained by microscopic fluorescent observation. The sites of photosensitizer localization were deter mined by organelle fluorescent probe, Rhodamine® 123, to specify the mitochondria location. The myocardial cell response to the photosensitization reaction was studied by the observation of intracellular Ca2+ dynamics. Intracellular free Ca2+concentration in the myocardial cells was measured with fluorescent Ca2+ indicator, Fluo-4 AM, by a high-speed Nipkow type confocal laser microscope during the PDT when the photosensitizer was distributed outside the cells or localized to intracellular compartments. We concluded that the Ca2+ influx accompanied by cell membrane damage when the photosensitizer was distributed outside the cells or on the cell membrane was the possible mechanism of the myocardial electrical conduction block with the photosensitization reaction.