Parenteral admn. of Butazolidin to rats in single doses of 1-40 mg. depressed the uptake of I131 by the thyroid. Rats given this drug daily for 10-16 days showed normal I131 thyroid uptakes after the last injns. of the drug. Paper chromatograms suggest that Butazolidin combines with iodide and thus competes with the thyroid for this element. Once the drug remaining in the body is in its equilibrium with iodide, the thyroid accumulation of I is resumed.