Photodynamic therapy is a minimally invasive cancer treatment modality based on the production of the reactive oxygen species by photoactive dye under light irradiation in the presence of molecular oxygen. During the development of the photodynamic reaction, various types of reactive oxygen species are formed, among which hydrogen peroxide is of the greatest interest since it can act as an extracellular and intracellular signaling molecule. Using a genetically encoded sensor of hydrogen peroxide, we have registered the development of oxidative stress in non-irradiated cells in response to local photodynamic exposure of a single cell using Photosens as a photosensitizer. The effect manifested when the cells were closely contacted to each other; if the irradiated cell was at some distance from the bulk of the population, the response of non-target cells was not observed. The oxidative stress in the irradiated cell is assumed to be the initiator of the signal transmission and triggering the response of non-target cells. That this response is more likely mediated by gap junction intercellular signaling. Нowever, the mechanisms involved in the propagation of damaging effects to cells outside the area of photodynamic exposure have to be further investigated.