Differences in the I-131 uptake by 14 metastatic lesions from well-differentiated thyroid carcinoma and 12 local remnants of normal thyroid tissue before and after an adjuvant therapy with lithium carbonate, were observed. After the adjuvant treatment and administration of an I-131 tracer dose, a considerable increase of radioiodine uptake in all metastatic lesions was found (P less than 0.001), but only a slight increase was found in 50% of the normal tissue. The response to lithium carbonate by neoplastic tissue seems to be different than that of the normal thyroid tissue, and produces a lengthening of the average I-131 biologic life that could be helpful when treating well-differentiated thyroid carcinoma.