Rabbits were fed a diet containing 3% cholesterol for 2–6 months. Total cholesterol, cholesterol ester, phospholipids, and triglycerides were increased in the plasma. Total cholesterol and cholesterol ester were also increased in the heart mitochondria and supernatant, and liver mitochondria, microsomes, and supernatant, but triglycerides and phospholipids in all cell fractions studied remained within the normal ranges. The respiratory control ratio, P/O ratio, and oxygen consumption of the cardiac and hepatic mitochondria were not affected by the increase in cholesterol content. Possible mechanisms responsible for the altered lipid composition in the cells of rabbits on a high-cholesterol diet, as well as that for the unchanged mitochondrial respiratory function are discussed.