To investigate the mechanism of cell photostimulation, we measured oxygen consumption of mitochondria during low-intensity laser irradiation at various wavelengths in vitro. Isolated guinea-pig liver mitochondria were irradiated with a low-intensity argon laser irradiation at the wavelength of 351 nm, 458 nm, and 514.5 nm. Both 351 nm and 458 nm irradiations increased the mitochondrial oxygen consumption on state 4 respiration in spite of no effect on state 3 respiration. The 514.5 nm irradiation activated the mitochondrial oxygen consumption on state 3 respiration in spite of no influence on state 4 respiration. These results might be explained that the ultraviolet irradiation causes damage to the mitochondrial inner-membrane, and the 514.5 nm irradiation enhances ATP-synthesis.