Recently old rats were found to be much more susceptible than young rats to the hepatocarcinogenic effect of a 55-59-week treatment with the peroxisome proliferator nafenopin (NAF) (B. Kraupp-Grasl, W. Huber, H. Taper, and R. Schulte-Hermann, Cancer Res., 51: 666-671, 1991). In the present study indicators of oxidative stress were measured in the livers of the same animals (male Wistar). NAF enhanced peroxisomal beta-oxidation 10-12-fold and reduced glutathione peroxidase activity by 40-50%. Indicators of lipid peroxidation like thiobarbituric acid reactive substances and malondialdehyde were both decreased by one-third and two-thirds, respectively. Of the oxidation-sensitive polyunsaturated fatty acids linoleic acid and docosahexaenoic acid were decreased by 40% and two-thirds, respectively, but the particularly sensitive arachidonic acid remained unchanged. Taken together these data suggest that NAF did not significantly enhance lipid peroxidation in the present experiment. All NAF effects were of the same magnitude in the old and young animals. Therefore, the considerably stronger induction of hepatocarcinoma by NAF in the old animals was not associated with evidence of enhanced oxidative stress. These findings are consistent with the hypothesis that NAF acts hepatocarcinogenically by promotion of tumor development from preneoplastic lesions occurring spontaneously with age.