The second order rate constant for the reaction between ascorbic acid and superoxide at pH 7.4 using the xanthine-xanthine oxidase system was estimated to be 5.4 x 106 M-1sec-1. The results indicate that the efficacies of superoxide dismutase and ascorbic acid for catalyzing the decay of superoxide radical in animal tissues are similar. The significance of ascorbic acid as a scavenger of superoxide is discussed from the point of view of evolution of ascorbic acid synthesizing capacity in terrestrial vertebrates.