Female F-344 rats were fed a diet containing up to 1.2% di(2-ethylhexyl)phthalate (DEHP) for 2 years, which previously resulted in hepatocarcinogenesis under bioassay conditions. Peroxisome proliferation, decreased glutathione peroxidase activity, and lipofuscin accumulation were all associated with prolonged feeding of 1.2% DEHP and induction of hepatic neoplasia. These results establish a potential role for persistent peroxisome proliferation and oxidative injury in the hepatocarcinogenicity of dietary DEHP. Increased hepatocellular proliferation and hepatomegaly were not detected. DEHP feeding did not increase the volume density of basophilic or ATPase-deficient foci of altered hepatocytes, suggesting that these lesions are not suitable indicators of DEHP carcinogenesis.