1. The action of potassium iodide in preventing a significant rise in the blood cholesterol of rabbits fed cholesterol was temporary. After about 4 months it lost its effectiveness and the blood cholesterol rose.
2. In rabbits with hypercholesterolemia resulting from long continued cholesterol feeding, the administration of potassium iodide caused a marked rise in the blood cholesterol.
3. On the other hand, dried whole thyroid given to such animals produced a sharp fall in the blood cholesterol. This fall was temporary and was followed by a rise to new high levels.
4. In thyroidectomized rabbits fed cholesterol and potassium iodide, both thyroid and thyroxin delayed but did not prevent a rise in blood cholesterol. Even with the hypercholesterolemia in these animals, however, the incidence of atherosclerosis was low.
5. Age apparently played a part in determining the response of the blood cholesterol to cholesterol feeding. In a group of old rabbits when compared with a younger group the rise in the cholesterol of the blood was greater and the subsequent return toward normal was slower when the feeding was stopped.
