The ability of thyroid tissue from non-toxic goiter to synthesis thyroid hormones was investigated by measuring 131 I-iodide incorporation in T4 in dispersed thyroid cell system. Dispersed thyroid cells were prepared by trypsinization from 5 pathological and 2 Normal thyroid specimens. The patients with non-toxic goiter were euthyroid or in mild hypothyroidism. The capacity of T4 synthesis was decreased in 4 cases and increased in 1 as compared to that of normal thyroid tissue. In one case, the impairement of T4 synthesis seemed to be related to the presence of thyralbumin as the major thyroid iodoprotein. In the 3 other cases with a decreased T4 synthesis, a good correlation was found between the capacity of T4 synthesis in dispersed thyroid cells and the thyroid activity in vivo (6 hr - 132 I-uptake). The discrepancy between in vivo and in vitro data in the case showing an increased T4 synthesis could be explained by the difference in the iodide supply in vitro (5 X 10(-8) M) and in vivo (urinary iodide : 2 microgram/24 hr). Thyroid cell preparations may serve as an experimental model to confirm or invalidate the hypothesis of a defect of thyroid hormone synthesis in non-toxic goiters.