Eighteen male guinea pigs were divided into two groups and fed for 2 weeks with a diet containing 284 ppm of vitamin C and 50 ppm of vitamin E. One group received autoxidized linseed oil (40 mg/animal/day) three times during the feeding. Compared with the animals not given the oxidized oil, the body weight gains, liver ascorbic acid levels and a-tocopherol contents were significantly decreased and liver lipid peroxidation was greatly stimulated in the animals dosed with the oxidized oil. In the next experiment, forty male guinea pigs were divided into five groups; only one group received a diet containing 1000 ppm of vitamin C and 50 ppm of vitamin E without oxidized oil, and the other four groups received graded diets of 1000 ~ 3000 ppm of vitamin C and 50 ~ 200 ppm of vitamin E for 35 days. To the latter four groups, the oxidized oil (24 mg/animal/day) was orally administered 12 times during the feeding period. The tocopherol contents of the kidney and lung were found to be lower in the animals fed with a 3000 ppm-vitamin C and 200 ppm-vitamin E diet than those fed with a 1000 ppm-vitamin C and 200 ppm-vitamin E diet. The lung ascorbic acid content was lower in animals fed with a 3000 ppm-vitamin C and 200 ppm-vitamin E diet than in those fed with a 3000 ppm-vitmin C and 50 ppm-vitamin E diet. The effect of dietary vitamins C and E on tissue lipid peroxidation was different among the individual organs. From the results obtained by chemiluminescence analyses, the lipid peroxidation caused in the liver, kidney and heart was recognized to be suppressed effectively when 200 ppm-vitamin E diets were given. Antioxidative synergism of vitamin E (200 ppm) and C (3000 ppm) was found in the liver and lung as estimated by the chemiluminescence.