ENDEMIC goiter has been thoroughly investigated with the aid of radioiodine by American and Argentinian workers (Stanbury et al., 1, 2). It has been shown by them that, in patients with goiter from the region of Mendoza in Argentina, the thyroid gland exhibits an increased avidity for iodine, with a rapid turnover which can be further accelerated by administration of thyrotropin. These findings are consistent with the theory which implicates iodine deficiency as the chief factor in the genesis of endemic goiter and eliminates, at least in the region studied, the possibility of a goitrogenic factor which would block the trapping of iodine by the thyroid gland. We were interested in confirming the finding of increased iodine avidity by studies in another endemic goiter area, and in ascertaining whether or not there existed a difference in thyroidal radioiodine uptake between goitrous and nongoitrous subjects from the same region. Although, in previous studies, no defect in the “iodine trapping mechanism” of the thyroid had been shown to exist, it remained to be seen whether the organic binding of the trapped iodine was unimpaired. Finally, the magnitude of I131 incorporation into plasma protein had to be studied, since Stanbury and his collaborators were unable to do so because they lacked sufficiently sensitive apparatus at the time of their study. In the present investigation we were able to explore this last point only in a preliminary way.