Radioisotopes of I pose an important health risk to man in nuclear accidents associated with electric power generation due to their uptake by the thyroid glands. Topical application of tincture of I or povidone-iodine to the skin of rats has been found to be as effective as oral administration of potassium iodide in blocking thyroid uptake of parenterally administered 131I. If the same effectiveness can be demonstrated in humans, this may be an attractive alternative method of mass protection from radioisotopes of I following nuclear accidents.