This study was undertaken to elucidate the toxic nature of dietary hydroperoxides in rats by the measurement of tissue chemiluminescence (CL). By the oral administration of methyl linoleate hydroperoxide (HPO) for two days, the tissue CL intensities and thiobarbituric acid (TBA) reactants were significantly increased in the liver, kidney, heart and lung, and the increases of both indices were inhibited by the supplemental administration of antioxidants, i.e. d-α-tocopherol (TOC) and riboflavin-tetrabutyrate (RTB). TOC was especially effective on the liver and kidney and RTB was effective on the liver, lung and heart. Hepatic glutathione peroxidase activity was evidently elevated at the seventh day of HPO-feeding, and this activation resulted in the decay of CL intensities and TBA reactants in rat organs. The spectrum of CL from liver homogenates of rats dosed with HPO for two days revealed five emission bands in the visible region and was similar to that for a singlet oxygen. The results indicate that TOC and RTB act as protective agents against the tissue lipoperoxidation caused by dietary HPO in rats, and that liver glutathione peroxidase is effectively induced as one of the bio-antioxidative functions in rats against the toxic effect at an early stage of the HPO dosage.