Free radicals are widely recognized as harmful chemical species in oxidative tissue injury. However, there have been no satisfying methods to visualize free radicals in vivo non-invasively with information of their localization and amount. In vivo electron spin resonance (ESR) spectroscopy was recently developed to measure free radicals generated in rodents. Several kinds of stable nitroxyl radicals were used as spin probes to detect free radicals. ESR signal intensities reflecting the accumulation of nitroxyl probes in each organ decreases time-dependently and reduction decay rates are increased in the presence of free radicals. Such increase in signal decay rates is suppressed by prior administration of antioxidants or antioxidant enzymes. Thus, in vivo ESR techniques are useful in estimating not only in vivo free radical reactions but also the effects of antioxidants, and furthermore, in combination with other tomographic techniques, permits non-invasive localization of free radicals. Application of this technique to animal models will be described.