The marked fall in plasma protein bound iodine (PBI) effected by dinitrophenol (DNP) is confirmed. DNP administration results in a decrease in (1) the thyroidal 24-hour uptake of I131; (2) the thyroidal iodide concentrating capacity and (3) the ratio of plasma PBI131 (conversion ratio) 24 hours following I131 administration. The rate of release of thyroidal organic I into the circulation is decreased and the rate of disappearance of thyroid hormone from the circulation is increased by DNP administration. It is felt that the rapid fall in plasma PBI effected by DNP can be readily understood in terms of these rate changes. The relationship of the actual level of circulating thyroid hormone to the output of thyroid stimulating hormone (TSH) by the anterior pituitary is discussed; it is suggested that factors other than the plasma PBI influence TSH output by the pituitary beta cells.