Hypophysectomized rats were injected with 25μc. of 131I, and the iodine uptake of the thyroid was followed with a Geiger counter for several days. At post-hypophysectomy intervals of 18–59 days no 131I was concentrated in the thyroid. An anterior pituitary extract ('Preglandol', Hoffmann-La Roche) in daily doses of 0·2 ml. given for 2 or 3 days, restored the activity of the thyroid. Thyrotrophin (Armour), a highly purified preparation, in quantities of 1–10 mg/day also restored the thyroid activity after 2–3 days. One or two daily injections of 2·5 mg. cortisone, administered simultaneously with Preglandol or thyrotrophin, prevented this activation of the thyroid. It is concluded that the effect of cortisone is due to a direct inhibition of the thyroid, and not to interference with the production of thyrotrophic hormone.