A novel Electron Paramagnetic Resonance technique has been developed to image physiologic characteristics and toxic radicals in living tissues of animals and humans. This information has, heretofore, been unavailable. Recent advances in spectroscopic technique and spin probe development, as well as the understanding of the information that can be provided, poise this technique on the verge of breakthrough. The technique uses very low frequency (100 to 300 MHZ) electron paramagnetic resonance imaging. Very low frequency is necessary to allow the electromagnetic energy, which stimulates resonant absorption, to penetrate deep into the tissues of animals. We develop Center facilities with an array of EPR imagers capable of providing real time information on tissue microenvironment. The Center functions within a major medical research facility. This offers adjacent, coordinated animal care, access to MRI, CT, PET and optical scanning and IMRT irradiators.