Oxidized frying oil (OFO) activates peroxisome proliferator-activated receptor a (PPAR a) in vitro and in vivo. As most PPARa activators are also Peroxisome proliferators (PP), this study was aimed at exploring whether OFO induces peroxisome proliferation in the liver of rats. Four groups of male weanling Sprague-Dawley rats were fed the following diets for 6 wk: a basal diet containing 5g/100g fresh soybean oil (LSB), high-fat diets con taining 20g/100g of fresh soybean oil (HSB as a control), OFO (HO) or fish oil (HF, as a pos itive control). Hepatomegaly and peroxisome proliferation in the liver of the HO group of rats were higher than those of the HF group. In addition, the acyl-CoA oxidase (ACO) activity, as well as cytochrome P₄₅₀ 4A (CYP4A) protein content in the livers of the HO group were 6 fold those of the HSB group, but were 2.5 fold in those of the HF group. These results indi cated that dietary OFO induced typical responses to PPARa signaling. Moreover, as a dietary source, the OFO prepared under our frying conditions appears to be a more potent peroxi some proliferator than fish oil.