Studies have been performed on groups of mini-pigs 21-23 months of age, which after 18 months of hypercholesterolemia (approximately 10 mmol) had developed raised atherosclerotic lesions with high levels of cholesterol esters, especially in the abdominal aorta and the coronary arteries. If the hypercholesterolemia was continued for 18 months, no significant change in the cholesterol ester content in the aorta occurred; in the coronary arteries there was a significant decrease in these older pigs. If the hypercholesterolemic pigs also were treated with beta-pyridylcarbinol the findings were very similar to the first. When hypercholesterolemic pigs were treated with clofibrate, or when the hypercholesterolemic diet was replaced with the basal food for 18 months, the plasma cholesterol level was normalized (approximately 2 mmol) within 1-2 months. The cholesterol ester content in the thoracic aorta was reduced in both groups but not that in the abdominal aorta. Clofibrate decreased the cholesterol ester level in the coronary arteries when compared to the hypercholesterolemic group; the drug also reduced the free cholesterol level when compared to the basal group. We suggest that an increased plasma cholesterol level initiated the development of the atherosclerotic lesions; their later development was only partly dependent on the plasma cholesterol level.