Conjugated linoleic acid (CLA) is reported as having several beneficial effects including anticarcinogenic, cholesterol-lowering and anti-atherogenic properties; however, CLA has also been reported as a putative peroxisome proliferator in mice. In this study the ability of CLA to cause peroxisome proliferation in the rat, as measured by accepted enzyme markers was investigated. Male Wistar rats were fed a semi-purified diet containing 0.0, 1.5 or 5.0 energy % CLA for 4 weeks. A positive control group were given 250 mg clofibrate/kg by gavage for 4 days. Hepatic cyanide-insensitive palmitoyl coenzyme A (PCoA) oxidase and carnitine acetyl transferase (CAT) activities and total cytochrome P450 (CYP) levels were measured. CLA had no effect on body weight or liver/body weight ratios, but clofibrate significantly increased mean liver/body weight ratio by 41.6%. Clofibrate-treated rats showed typical changes with increases in hepatic PCoA oxidase and CAT activity (5.8-fold and 22.8-fold) and in total CYP (1.66-fold) compared with control. There were no differences between the control group and the groups fed CLA for either the peroxisomal enzymes or total CYP. These results suggest that CLA does not act in the rat as a classical peroxisome proliferator and that there may be a species difference in the effects on rat and mice.