The renewal of catalase activity in C57BL/6J male mice of different ages was followed for 24 h after the removal of activity with 3-amino-1,2,4,-triazole (AT). The return of activity after injection of AT in both liver and kidney of old animals was significantly slower than that of either young or middle-aged mice. Some old mice failed to renew any catalase activity following treatment with AT. This suggests either an increase in the rate of degradation, or decrease in the rate of synthesis, of catalase in old mice, since all old animals showed catalase activity prior to treatment.The study demonstrates that the regulation of catalase is a good model system for age-related studies of enzyme regulation. In addition, the possible involvement of catalase as an intrinsic factor in senescence is discussed.