RATS fed a low iodide diet until goiters developed utilized so little iodide that chemical methods were unsatisfactory for determinations of plasma PBI or of urinary and fecal iodide. A method was developed (Van Middlesworth, 1952) which involved feeding rats an I131-tagged diet until an equilibrium was established between the specific activity of the diet and the specific activity of all iodide pools of the animal. Then the I131 in any pool or compound of the animal was assumed to be proportional to the amount of chemical iodine in that pool, regardless of the relative rates of labeling. METHODS Young adult male Long-Evans rats were used throughout these investigations. The animals were maintained for 4-6 months on a low iodide, goiter-producing diet (Van Middlesworth, 1955) before they were used in these experiments.