The main objective of this study was to investigate the possible cause(s) of the age-related augmentation of oxidatively damaged proteins in animal tissues. The hypothesis that activity of alkaline proteases, involved in the proteolysis of oxidized proteins, declines during aging was tested in the adult male housefly and further explored in the rat. Alkaline protease activity was measured fluorometrically by the release of trichloroacetic acid-soluble fluorescamine-reactive material from X ray-oxidized bovine serum albumin (BSA). Alkaline protease activity in the housefly was linearly related to the number of protein carbonyl groups. Possible involvement of serine or serine and thiol proteases was deduced from a 70% proteolytic inhibition by aprotinin and a 50% inhibition by leupeptin. Protease activity of houseflies for oxidized or native BSA did not alter with age. In contrast, a varied age-related pattern of protease activity was observed in the tissues of the rat. A comparison of 3-, 13-, and 23-month-old Sprague-Dawley rats indicated no age-related decline in alkaline protease activity in the brain, a 50% decline in the liver, and a 20% decline in the heart during 13 to 22 months. Results of this study suggest that in some species or tissues an age-related increase in the oxidized protein content is primarily due to a corresponding increase in the rate of protein oxidation, while in some other tissues a decline in proteolysis may be a contributory factor.