Marked species differences in thiobarbituric acid reactant value (TBA value) in normal lung tissue of five species of animals were found. The order of the values was mouse greater than hamster greater than rat greater than guinea pig greater than rabbit, and the value for mice was 3.6 times higher than that for rabbit. The vitamin E (VE) and nonprotein sulfhydryls (NPSH) contents in lungs varied widely among the five animal species. Species differences were also observed on polyunsaturated fatty acid composition in lung phospholipids. The peroxidizability index (PI), which shows the relative rate of peroxidation reaction, was calculated from the composition ratio and the reactivity of each polyunsaturated fatty acid, and the PI was found to be significantly correlated to the TBA value in lungs (r = 0.853, p less than 0.001). The PI value was normalized by the contents of VE and/or NPSH. Finally, the log-value of PI, normalized by the log values of the reciprocals of VE and NPSH, log(PI/VE X NPSH), showed the highest correlation coefficient (r = 0.907, p less than 0.001). Normalization by the activities of antioxidative protective enzymes in lungs did not show any significant correlation against TBA value. These results suggest that TBA value as an index of lipid peroxides in the lungs of animals may be regulated mainly by the contents of VE and NPSH, the composition ratio and the reactivity of each polyunsaturated fatty acid in lung phospholipid fraction.