The hyperlipidemia is a well-known typical symptom in Yusho patients and experimental animals treated with PCBs. We have found a significant induction of CYP4A1, which catalyzes ω-hydroxylation of fatty acids, in guinea pigs by the treatment with a coplanar PCB, 3,4,5,3',4'-pentachlorobiphenyl (PenCB), though the P450 is reduced in the treated rats. Peroxisome has β-oxidation enzymes distinct from mitochondrial enzymes, and also play an important role in lipid metabolism. Peroxisome proliferators have been shown to regulate the expression of CYP4A1 and peroxisomal enzymes by the same mechanism in the rat. In the present study, we examined the effect of PenCB treatment on peroxisomal enzymes in the liver of guinea pigs. As a result, the enzyme activities of hepatic peroxisome, e.g. fatty acid oxidizing system, catalase and urate oxidase, had a rising tendency by the treatment with PenCB in the animal. The results suggest that the regulation of peroxisomal enzymes and CYP4A1 is also associated in guinea pigs, and PenCB provides a similar effect of peroxisomal proliferators to the animal. The possible toxicity through the peroxisomal alteration was discussed.