A number of thyroxine analogues have been administered to rats, and their effect on the uptake of radioactive iodine has been investigated. The marked depressions nearly always obtained can in many cases be attributed to iodide liberated by decomposition of the drugs. A marked sex difference was observed with certain compounds. In long-term experiments in which the drugs were incorporated in the diet alone or together with thiouracil, the growth rate, thyroid weight and thyroid total iodine were determined.