This study was conducted to investigate the protective role of vitamin E against the formations of lipid peroxides in plasma and tissues and of 8-hydroxydeoxyguanosine (8-OHdG) in livers of rats fed fish oil. Six-week-old male Sprague-Dawley rats were divided into four groups and fed experimental diets for 8 weeks. Three fish oil (F) groups were fed a menhaden fish oil and soybean oil (9:1) mixture as 10% (wt/wt) of their diet. These three groups (F0, FI, and FII) were provided with < or = 3, 45, and 209 IU of vitamin E/kg diet, respectively. The SI group was fed soybean oil with < or = 45 IU of vitamin E/kg diet. The F0 group had the highest levels of thiobarbituric acid-reactive substances (TBARS) in plasma (per milligram lipid), and liver, lung, heart, and kidney. The FI group had higher levels of TBARS than the SI group in plasma and tissues except the lung. In liver, the TBARS levels of the FII group were also higher than those of SI group, but in other tissues, similar levels were observed in the FII and SI groups. Plasma level of vitamin E was lowest in F0 group and vitamin E levels were generally lower in F groups than in SI group. These levels were expressed as vitamin E per milliliter of plasma. However, plasma E levels were similar when expressed per milligram plasma lipid. The liver 8-OHdG concentration tended to decrease as dietary vitamin E increased in the F groups, but there was no difference in the level of 8-OHdG between the FI and SI groups. These results suggest that vitamin E should be supplemented in fish oil feeding to prevent the enhanced lipid peroxidation and the formation of 8-OHdG in the body.