Erythrosine (tetra-iodofluorescein), an approved food dye, is used in pharmaceutical products and in some foods as a coloring agent. In three separate experiments an erythrosinecontaining cereal was fed to rats for 3–5 weeks. At the end of each study the distribution of 131I and 127I in thyroid, serum and urine was studied. The effect of erythrosine was to decrease 24 hr 131I uptakes by the thyroid gland and increase total thyroidal 127I. In serum the protein-bound 131I was decreased and there was an increase in the non-protein-bound 131I in erythrosine-fed animals. Urinary 131I was increased in erythrosinefed animals and total 127I and urinary iodide were sharply increased. Erythrosine was labelled with 131I and in all three experiments it was shown that one-fourth to one-third of the label was metabolized to iodide in the rat. In view of its widespread use, it is concluded that ingestion of erythrosine-containing foods or drugs can contribute substantially to dietary iodine intake.