Lithium as a trace element has assumed recent importance because of its role in the treatment of manic depressive psychosis, and in the genesis of atherosclerosis and goitres. Lithium has important effects in many different organ systems, particularly those involved in ion transport and polypeptide hormone action, and it is also an effective therapeutic agent in manic depressive psychosis. There is a vast difference between the large amounts of lithium required therapeutically and the small amount of lithium which is consumed regularly in the diet.