Uric acid has previously been shown to act as a water-soluble antioxidant. Although the antioxidant activity of uric acid has been attributed to its ability to scavenge free radicals, the one-electron uric acid oxidation product of such a scavenging reaction has not been detected. It order to determine whether a free radical metabolite of uric acid could be formed via one-electron redox processes, we oxidized uric acid with potassium permanganate, horseradish peroxidase/hydrogen peroxide, and hematin/hydrogen peroxide systems. With the use of the rapid-mixing, continuous-flow electron spin resonance technique, we were able to detect the urate anion free radical in all three radical-generating systems. Based on N15-isotopic-labeling experiments, we show that the unpaired electron of this radical is located primarily on the five-membered ring of the purine structure. We were also able to demonstrate that this radical could be scavenged by ascorbic acid.