In rabbits fed an atherogenic semi-synthetic diet, the following were found: in the aortic wall, decreased cholesterol esterase and increased lipase and phospholipase A activities, no changes in malate and lactate dehydrogenase activities, enhanced incorporation of free fatty acid into cholesterol esters, and increased accumulation of mainly esterified and also free cholesterol; increase in plasma cholesterol and serum phospholipase A activity but no change in liver phospholipase A and serum and liver lipase activities. Compared with control animals, those fed the same diet and injected with Lipostabil (a drug containing polyunsaturated lecithin) showed no alterations in the aortic enzyme activities and serum phospholipase A and in the incorporation of free fatty acid into aortic cholesterol esters; serum and liver lipase activities were increased. Compared with atherogenic diet-fed animals injected with saline, the severity of atherosclerosis and the incorporation of free fatty acids into the aortic wall were reduced; there was no change in the elevated plasma cholesterol levels. The mechanism of cholesterol ester accumulation in the arterial wall is discussed in the light of these observations.