The effect of an increased iodide level in the blood for one week on the iodine metabolism of the thyroid was investigated in 28 patients with nontoxic goiter by means of isotope and chemical analyses. The average initial values of 0.8 μg/100 ml blood iodide and of 14.3 μg/h absolute iodine uptake of the thyroid did not deviate from the previously found normal values. Increasing the iodide concentration in blood caused at first stimulation and then inhibition of the thyroid iodine uptake. Normally its complete block was reached at 30.0 μg/10 ml, whilst for the nontoxic goiter this did not occur below 100.0 μg/100 ml blood iodide. An increased thyrotrophin-effect and not the larger thyroid volume is responsible for this difference. The secretory activities of the nontoxic goiter and of the normal thyroid were equally little influenced by iodine administration.