ABSTRACT An extensive review of the thyroid hormone by Pitt-Rivers & Tata (1959) indicates that the principal if not the only urinary metabolite of thyroid hormone is inorganic iodide in normal rats. In a series of normal and hyperthyroid rats injected with 131I labeled triiodothyronine (T3–131I) this was not observed to occur. Within a short time of injection of a tracer dose of T3–131I, appreciable amounts of T3–131I, mono and diiodotyrosine were detected in the urine by ascending paper chromatographic separation of urines collected from the bladder at the time of killing the animals. During the earlier experimental periods (1/2, 1 and 2 hours) organic iodine made up a large percent of the excreted 131I found in bladder urine. It was only by the seventh hour that inorganic iodide (131I) appeared in really appreciable amounts. The hyperthyroid state was observed to be accompanied by a decreased total excretion of 131I via the kidneys. However, the percent of urinary 131I present as organic iodine in hyperthyroid rats reached a maximal amount earlier than in normal rats. Comparison of the 131I labeled materials in bladder urine with those in the plasma from the same animals suggested that the nature of the excreted 131I labeled material was in general closely related to what was present in the plasma. Thus, when plasma iodide reaches high levels seven hours after injection of T3–131I, urinary excretion of free iodide becomes highest.