The hydrogen peroxide-dependent and peroxidase-mediated iodination of tyrosine, a pivotal step in the thyroxine biosynthesis in thyroid glands of mammals, has always been thought to be dependent on a glucose-glucose oxidase system to meet its demand for hydrogen peroxide. This report documents a novel and probably kinetically faster hydrogen peroxide-generating system operating in the thyroid gland cells involving the superoxide dismutase and superoxide anion radical. In the female cycling mice, the superoxide radical and superoxide dismutase activity are shown to be at persistently high levels, quite likely acting as a potent hydrogen peroxide synthesizer in the mammalian thyroid gland. The significance of this discovery in relation to the thyroid physiology is discussed.