The influence of iodide and iodized compounds upon radioactive iodine tracer tests and on the PBI was investigated by repeated tests in 75 patients. The substances were contrast media for cholecystography and intravenous pyelography, iodized poppy seed oil ("Lipiodol"), iodochlorhydroxyquinoline ("Entero-Vioform"), iodocasein ("Etibol"), and a colloidal iodine preparation ("Tikajod"). The series comprises patients with thyrotoxicosis and with atoxic nodular goiters, patients treated surgically or by radioiodine for thyrotoxicosis, one patient with juvenile atoxic goiter, one who was hypothyroid following radioiodine therapy and, lastly, patients with no known thyroid disease. An attempt was made to determine the time required for normalization of the tests following administration of the various agents and also to ascertain whether the duration of the influence varied with different biological states of the thyroid. Although the series as a whole exhibits major variations, it is possible to establish for each compound a maximum duration after which it fails to influence tracer tests and PBI. The tracer test is frequently influenced for a longer period than is PBI. Patients with thyrotoxicosis and, possibly, those with diffuse juvenile goiter are affected for considerably lesser periods than are euthyroid subjects.