Changes in the oxidation state of biological systems as a function of ionizing radiation were studied by measuring the activity of two metalloenzymes and by evaluating electron spin resonance signals produced by mitochondria. The specific activity of liver cytochrome oxidase and xanthine oxidase was not altered in mice exposed to total-body irradiation of 150 to 22,500 rad. Iron (Fe(II)), molybdenum (Mo(V)), and free radicals (probably flavoquinones) yielded the most predominant signals in electron spin resonance analysis of liver mitochondria isolated from both irradiated and control mice. Both the electron spin resonance and cytochrome oxidase data suggest that the electron transport system is not damaged by the levels of ionizing radiation used in this investigation. The fact that molybdenum electron spin resonance signals occur in the mitochondrial preparations and that preliminary data indicate this metal undergoes oxidative changes due to radiation indicates the need for more extensive investigation of the relationship between this metal and ionizing radiation. (Author).