We have previously described an aging-related decrease in the peroxisomal polyunsaturated fatty acid oxidizing system in mouse liver. In order to determine whether peroxisome synthesis is involved in this phenomenon, we focused our work on different peroxisomal enzyme activities during aging in the liver of mice fed for 5 days with either a control or a clofibrate supplemented diet which enhanced peroxisome biogenesis. Liver peroxisomal acyl-CoA oxidase (AOX), catalase (CAT) and urate oxidase (UOX) activities per gram of liver were determined. In control mice, UOX activity was not affected by aging whereas CAT and AOX activities were significantly decreased. At day 300 the clofibrate treatment increased all activities although UOX was not significantly increased. Thereafter, enzyme activities after clofibrate treatment were severely depressed at day 680. CAT and UOX were not induced in very old clofibrate-treated animals, whereas AOX was induced 7 fold in such mice compared to an 11 fold induction in day 300 animals. The present results suggest that: 1- Aging decreased the peroxisomal polyunsaturated fatty acid oxidizing system. 2- This took place via a specific decrease in AOX activity. 3- Since clofibrate treatment triggers the peroxisomal proliferation, the aging-related decrease in peroxisomal activities might be due to an alteration in peroxisome synthesis.