Localized electron spin resonance spectroscopy in live mice was performed using a surface coil operating at 1.1 GHz with sufficient sensitivity and stability to measure quantitatively the time course of the distribution, uptake, and reduction of nitroxides in selected organs/regions (liver, bladder, head) of mice. The ability to measure regional concentrations of nitroxides in vivo could be used for pharmacokinetic analysis of drugs labeled with nitroxides and for measurement of oxygen concentrations and redox metabolism.