Light-induced Ca2+ concentration oscillations in cytoplasm of oat protoplasts were shown to be controlled by inhibitors of Ca^"*" pumps, sodium vanadate and tuBHQ. The sources of Ca^'*' released into cytoplasm under the action of radiant energy are vacuole and endoplasmic reticulum, but not mitochondria. The manifestation of Ca^'*' oscillations depends on the concentrations of oat protoplasts in the suspension which suggests the propagation of a signál between the cells and synchronization of their light-induced Ca2+ oscillation response.