The reactions of uric and 1-methyluric acids in nonpolar environments, as well as those of the corresponding urate anions in aqueous solution, with (center dot)OH, (center dot)OCH(3), (center dot)OOH, and (center dot)OOCH(3) have been studied using the density functional theory. Different mechanisms of reactions have been taken into account, and their relative importance on the antiradical activity of these compounds is analyzed. Both uric and methyluric acids are better scavengers in aqueous solution than in nonpolar media, which indicates that the urate anions are the most active species. The free radical scavenging activity of the studied compounds was found to be excellent for (center dot)OH, and very good for (center dot)OCH3. In addition, 1-methyluric acid is predicted to moderately protect against peroxyl oxidation, while the protective effects of uric acid against these particular species are not expected to be significant. In addition, 1-methyluric acid was found to be a better radical scavenger than its precursor, caffeine, suggesting that the antiradical activity of the latter might be explained by the action of its metabolites, rather than by its direct activity.