The quantity of free malondialdehyde (MDA) in liver tissues of rats fed vitamin E-deficient or-supplemented diets for 43 wk was measured by a newly developed high performance liquid chromatographic (HPLC) method. Bound MDA was quantified by the same HPLC method after alkaline hydrolysis of tissue homogenates. Tissues from vitamin E-deficient animals showed levels of free MDA about 15 times higher but levels of bound MDA less than 2 times higher than the vitamin E-supplemented animals. Free MDA is the major form in vitamin E-deficient tissues, but bound MDA is predominant in vitamin E-supplemented tissues. Conventional thiobarbituric acid (TBA) test results revealed that the content of TBA-reactive substances expressed in MDA equivalents was much higher than the actual free MDA levels in all groups. Results indicate that free MDA level measured by HPLC is a more sensitive index than the TBA value for lipid peroxidation. Some other TBA-reactive substances seem to exist in liver tissue regardless of the dietary treatment.