1. Clofenapate (methyl 2-[4-(p-chlorophenyl)phenoxy]-2-methylpropionate) fed to the rat in the diet increased the content of mitochondrial protein in the liver by 50–60%. In this respect it resembled the related compound clofibrate (ethyl α-p-chlorophenoxyisobutyrate), which is widely used as an antihypercholesterolaemic drug. 2. Both compounds when fed to the rat enhanced the activity of α-glycerol phosphate dehydrogenase in the liver mitochondria manyfold, but were without effect on the enzyme in the soluble fraction. 3. On the other hand, the catalase activity in the supernatant fraction increased twofold after administration of the drugs. The mitochondrial catalase activity showed a consistent decrease. 4. It was unlikely that under the influence of the drug the increase in catalase activity took place in the mitochondrial particles and was leached into the cytosol during isolation. 5. The increase in catalase activity in the cytosol under the influence of the drug is best explained on the assumption that peroxisomes which contain this enzyme, and which are known to increase on administration of the drug, were broken during the process of cellular fractionation and released the enzyme into the cytosol. 6. All the above effects shown by both drugs were fully reversed when drugs were withdrawn from the diet. 7. Clofenapate was effective in bringing about the above changes when administered to the animal at one-hundredth the concentration of clofibrate.