To clarify the intrinsic process of aging in aorta, the lipid metabolism was investigated in serum, aorta, and liver of 24-month-old rats in comparison to 10-month-old animals. Serum total cholesterol and the lipid content of low-density lipoprotein slightly increased in aged animals. Enzyme activities which hydrolyze or synthesize cholesterol ester or triglyceride in aorta were enhanced in aged animals. The increase in hydrolysis overcame that in synthesis, suggesting that enzyme activities were not changed in order to accumulate cholesterol ester and triglyceride in the aorta with aging. Phospholipid content and choline phosphotransferase activity were low both in the aorta and liver of aged animals. Moreover the phospholipid to free cholesterol ratio was decreased in aged animals, suggesting a change of membrane property with aging.