In an earlier paper it was suggested by one of us that the internal secretion of the thyroids may influence metabolism in the manner of a kinase, namely, by activating some intracellular oxidases.In view of a close parallelism existing between the iodine content of the thyroids and the physiological action of thyroid preparation it seemed possible that the administration of iodides might lead to an increased power to effect certain oxidative changes.To test this hypothesis, the capacity of tissue taken from normal dogs to oxidize indol was compared with that of tissues taken from dogs treated with potassium iodide (0.1 gram per kilo daily) over variable periods. The dogs were bled from the carotids, the organs removed under the usual precautions, hashed, divided into convenient quantities and exposed to seven times their weight of 0.005 per cent, indol solution. Toluol was added to prevent bacterial action. The mixtures were kept at body temperature, and the amount of unoxidized indol was determined at va...