Parenchymatous nodular, colloid nodular goiter, and mixed forms are found in the endemic goiter area of Cali, Colombia. Normal values of thyroid I131 uptake, PBI131 (protein-bound iodine-131) and PBI127 with a high stable urinary iodide excretion indicate sufficient iodine supply at the present. Intrathyroidal iodine metabolism was studied in pieces of nodular and paranodular tissue obtained from 19 goitrous, euthyroid, or hyperthyroid patients undergoing thyroid surgery after I131 administration. Iodide, mono- and diodotyrosine (MIT, DIT) and thyroxine (T4) were found in all tissue examined. The amounts varied depending on the length of time after I131 administration and on the type of the tissue. All tissues from colloid goiters and nodular tissue from parenchymatous nontoxic goiters showed decreased stable iodide and I131 concentrations, a higher MIT: DIT ratio and lower amounts of thyroxine when compared with tissue from normal glands. Tissue of toxic nodules contained more stable iodide and I131 and had a lower MIT: DIT ratio and higher thyroxine content than tissue from normal glands, or non-nodular tissue from the same gland. Serum con tained only T4 and iodide; urine, only iodide. In only one patient was there MIT and DIT in urine and serum. The alterations observed indicate a decreased thyroxine synthesis in the cold nodules and in the whole tissue of the colloid nodular goiters. Glands with less stable iodine tended to have higher MIT:DIT ratios. In the absence of iodine deficiency or its long-term effect, a goitrogen should be considered as a possible cause.