ALTHOUGH previous investigations have demonstrated that the thyroid gland of both euthyroid and thyrotoxic patients responds to administration of exogenous thyrotropin (1–7), the existence of differences in the nature of this response has not been explored fully. It was therefore considered of interest to make a quantitative study in normal and hyperthyroid subjects of the effect of varying doses of thyrotropin. In most thyrotoxic patients the thyroidal uptake of I131 is so high initially that the detection of any stimulation by means of this measurement is difficult. Of the parameters available for assessing thyroid function, we therefore employed the thyroidal secretion rate, as measured by the release of radioactive iodine from the thyroid gland, believing this to be the most sensitive. Use of the rate of thyroidal secretion as a parameter also obviates the necessity of giving repeated doses of radioisotope. In addition, corollary studies were made of changes in the serum protein-bound stable iodine and protein-bound I131 levels.