Hypolipidemic drugs increased 3- to 4-fold the activity of the peroxisomal beta-oxidation system in rat liver, with modest or no effects on catalase activity, liver weight, or peroxisome abundance. This specificity of action was observed in two experimental models: 1) bezafibrate treatment of male rats (25 mg/kg body wt., p.o.) and 2) clofibrate treatment of female rats (5 g/kg chow). Bezafibrate had no effect on the liver content of protein, catalase, or cytochrome oxidase, and little or no effect on mitochondrial beta-oxidation. The results indicate that the hypotriglyceridemic mechanism of action of these drugs involves an induction of the peroxisomal beta-oxidation system, but this mechanism does not obligatorily include gross hepatomegaly or other alterations of peroxisomes that are often caused by hypolipidemic compounds. This dissociation of specific biochemical changes from other effects demonstrates a precise regulation of organelle biogenesis. Peroxisomes synthesized under the influence of bezafibrate or clofibrate have a different enzymatic composition than do normal peroxisomes. These results have several implications. 1) Side effects of clofibrate that are of current clinical concern may be unrelated to its lipid-lowering effects. 2) Measurement of peroxisomal beta-oxidation should be a sensitive and specific tool for screening for new hypotriglyceridemic compounds. 3) Peroxisome proliferation or lack thereof is not central to efficacy. 4) Other new drugs may be discovered that are highly discriminating in elevating specific enzymes of fatty acid catabolism while causing even less or no hepatomegaly and other side effects.-Lazarow, P. B., H. Shio, and M. A. Leroy-Houyet. Specificity in the action of hypolipidemic drugs: increase of peroxisomal beta-oxidation largely dissociated from hepatomegaly and peroxisome proliferation in the rat.