Age-related changes in phosphatidylcholine hydroperoxide (PCOOH) content as an index for oxidative membrane lipid damage were determined by high-performance liquid chromatography using chemiluminescence detection. Brain and liver PCOOH content increased significantly in male and female rats with age. The brain PCOOH content of male 18-month-old rats was 4.4 times that of 1-month-old rats, and that of female 18-month-old rats was 3.5 times that of 1-month-old females. The liver PCOOH content of the male 18-month-old rats was 9.3 times that of the 1-month-old; and of the famale 18-month-old rats was 4.7 times that of the 1-month-old. PCOOH levels in heart and lung did not show age dependency. In both brain and liver (but not in heart and lung), the phosphatidylcholine content significantly decreased upon aging. The results indicate that oxidative deterioration, such as phospholipid hydroperoxidation, is prevalent in the membrane lipids of brain and liver of the rat due to aging.