Adult male mice (NMRI strain) were initially treated with 0.5% clofibrate in the diet during four days to obtain a high level of peroxisomes in the hepatocytes. Groups of animals were then given control diet for one, two, three, or four days. Liver samples were cytochemically stained for catalase prior to examination by electron microscopy and subsequent morphometric analysis. Various enzyme activities were assayed in liver homogenates. A sixfold increase in peroxisomal area was found after four days of clofibrate feeding compared with untreated animals. The peroxisomal fractional area was unchanged after one day of refeeding control diet but was reduced with over 50% after two days on control diet and was similar to that in the untreated animals after four days. The size distribution of peroxisomes shifted to smaller values during the breakdown of peroxisomes and was similar to the untreated animals after four days of clofibrate withdrawal. During this process a more heterogeneous staining of peroxisomes was observed. The mitochondrial fractional area was not affected by the treatments, but a shift in size distribution to smaller values was observed in the most induced animals. Protein content was not affected by the treatments. The activity of catalase (twofold induction) decreased progressively to control values during recovery but enoyl-CoA hydratase activity (fourty-fold induction) decreased rapidly. NADPH-cytochrome c reductase activity (twofold induction) decreased rapidly to control levels. Citrate synthase activity was not affected by clofibrate treatment but decreased during recovery. Acid phosphatase activity (repressed) increased to control levels. Cathepsin D activity increased somewhat during recovery while proteolytic activity (towards casein) decreased transiently on control diet.