The antioxidative effect of vitamin B_2 On the recovery of mitochondrial potential disorders induced by ultraviolet light exposure was studied by the method of fluorescence of 8-anilino-1-naphthalene sulfonate. Vitamin B_2 (free form) was injected intraperitoneally to rats during 6 days, after which mitochondria were prepared from rat liver. Ultraviolet irradiation upon the mitochondrial suspension resulted in the increase of fluorescence on the C-side of inner mitochondrial membrane. However vitamin B_2-administration was found to prevent the change of surface potential in the state of non-energized mitochondria and the respiratory disorders of mitochondria. The biochemical effect of vitamin B_2 to the surface potential of membrane was discussed by interaction with lipid peroxidation in mitochondria.