1. The daily feeding of 0.001 gm. KI to guinea pigs does not increase the number of mitoses and does not affect the structure of the thyroid gland. 2. Feeding daily 0.001 gm. KI to guinea pigs produces only a slight increase in mitoses but at the same time produces an otherwise histologically active thyroid gland. 3. The feeding of 0.01 gm. KI stimulates the thyroid in all its histological phases.