Free uric acid concentrations declined with aging in male Oregon R Drosophila melanogaster by 59% or more between 0 and 50 days of adult age. Free xanthine concentrations increased between 0 and 5 days of age and declined by 75% between 5 and 50 days of age. Xanthine oxidase activity was maximal for newly emerged flies and then declined rapidly reaching a minimum at 9 days of age. After 9 days of age xanthine concentrations may be the limiting factor for the production of uric acid by xanthine oxidase in aging fruit flies. Declining uric acid concentrations may represent a loss of antioxidant potential in aging Drosophila.