THE mechanism whereby iodide exerts its antithyroid action has been the subject of speculation from the time that its beneficial effect on Grave's disease was first noted. The phenomenon has been brought under experimental control and it is known that sufficient concentrations of iodide will completely block the formation of organically-bound iodine both in normal thyroid tissue and in surviving thyroid slices (Wolff and Chaikoff 1948; Morton, Chaikoff and Rosenfeld 1944). Two of the various theories advanced to explain this action of excess iodide are based on well-known chemical reactions of iodide ion. The first of these, which was advanced by Pitt-Rivers (1950) and Harington (1951), postulates that hypoiodous acid is the reactive form of iodine which enters organic combination in the gland. This substance is formed by the hydrolysis of elemental iodine: