1. The blood cholesterol of rabbits on a normal diet without added cholesterol is increased only slightly (19 per cent) by thyroidectomy.
2. In rabbits with a hypercholesterolemia due to long continued cholesterol feeding, thyroidectomy causes a marked rise (137 per cent) in the blood cholesterol. This rise is usually maintained.
3. When long continued cholesterol feeding has failed to cause a rise in the blood cholesterol of rabbits, thyroidectomy abolishes this resistance and a hypercholesterolemia is promptly produced.
4. A single injection of thyroxin causes a significant drop in the blood cholesterol of rabbits with hypercholesterolemia. This reaction is not influenced by thyroidectomy.
5. Potassium iodide causes an increase in the blood cholesterol of rabbits with hypercholesterolemia. This reaction is not influenced by thyroidectomy in contrast to the effect of KI in preventing a rise in blood cholesterol when given concurrently from the beginning of cholesterol feeding, an effect which is abolished by thyroidectomy. It is suggested that two different mechanisms are involved.
6. A single injection of insulin produces a fall in the blood cholesterol of rabbits with hypercholesterolemia. If anything, thyroidectomy increases the magnitude of this effect.