The effect of light irradiation on the absorption of rubidium was examined in Chlorella. Rubidium absorption in Chlorella was clearly stimulated by the irradiation of light. To clarify the mechanism of light-stimulation on rubidium absorption, experiments were carried out using several metabolic inhibitors; DCMU, NaCN,DNP,CCCP and ouabain. Among the metabolic inhibitors used, DCMU had the most similar effects on the rubidium absorption and oxygen evolution under light condition. These findings suggest a close correlation between the light-stimulation on rubidium absorption and the photosynthetic process in Chlorella.