Three groups of male rats were maintained on 10% fat diets containing 0.5, 5 or 10% corn oil or olive oil with 80--400 mg DL-alpha-tocopherol per kilogram. After 4 weeks on such regimens, TBA (thiobarbituric acid) values in serum, liver mitochondria and microsomes, and adipose tissue increased with rising amounts of dietary corn oil. TBA values in rats fed the 10% corn oil diet were reduced with the increase of dietary tocopherol but were still higher than the corresponding values of the 10% olive oil and 0.5% corn oil groups. When the liver microsomes were incubated with Fe3+-ADP and NADPH, the relative chemiluminescence emission in the visible region with the peroxidative cleavage of endogenous lipid was higher in the 5 and 10% corn oil groups than in the 0.5% group. On the other hand, when oxidized corn oil was given orally to rats with thoracic lymph fistula, TBA-reacting substances were recovered in thoracic lymph, but iodometric peroxide was undetectable. Therefore, TBA-reacting substances in rats fed the corn oil diets could have originated from the oxidative product of linoleic acid metabolism and also from the diet.