White Carneau pigeons were maintained for 26 1/2 months on cholesterol-free diets (pigeon pellets). When such diets were supplemented with 10% Crisco, aortic atherosclerosis and the level of aorta cholesterol were less than in birds receiving no supplement, or 10% safflower oil. Serum cholesterol levels were not different in these 3 groups. When other groups were maintained on diets containing 1% cholesterol for 6 months, the severity of atherosclerosis was increased, and inclusion of 10% safflower oil in the diets exerted no prophylactic effect. When cholesterol was removed from the diets, and the birds were maintained for an additional 12 months on either pellets alone, or pellets plus 10% safflower oil, there was no decrease in the level of aorta cholesterol and a slight decrease in aortic atherosclerosis.