The present study was designed to determine the effect of eicosapentaenoic acid (EPA) on the susceptibility of tumor cells to treatments that kill the cells by lipid peroxidation. Using AH109A carcinoma, a rat liver cancer, we measured EPA content, levels of antioxidants, and degree of lipid peroxidation in tumor tissue and normal liver tissue after oral administration of EPA. In the control group treated with distilled water, EPA in tumor tissue was lower than in normal liver tissue, suggesting that its content of polyunsaturated fatty acids (the substrates for lipid peroxidation) was inherently low. Levels of antioxidants also tended to be lower in tumor tissue. EPA level increased in both tumor and normal tissues after oral administration of EPA. At the same time, glutathione peroxidase (GSH-Px) increased in normal tissue, whereas tumor tissue displayed no increase in antioxidants; instead GSH decreased. The EPA-induced change in balance between substrates for lipid peroxidation and antioxidants suggested that tumor tissue might become more susceptible to lipid peroxidation than normal liver tissue. In fact, hyperthermia treatment did enhance lipid peroxidation and antitumor action. Our results indicate that oral EPA specifically increases the susceptibility of liver tumor tissue to lipid peroxidation, and hence enhance the antitumor effect of hyperthermia and prolongs survival.