The range of values for the 24-hour thyroidal accumulation of radioactive iodine in euthyroid persons varies with geographic location. In the San Bernardino Valley region of Southern California the "normal range" is 6 percent to 33 percent in euthyroid subjects. This is lower than in studies from other areas of the United States. The urinary iodide excretion and the absolute iodine uptake of the thyroid are higher than in studies from many other areas of the United States, pointing to iodine abundance as the reason for this difference. The geographic variation and the possibility of changing dietary iodine intake of normal persons point to the necessity of current and local determinations of the "normal range" of the thyroidal uptake of radioiodine if the results of this thyroid function test are to be properly interpreted.