The prophylactic administration of 100 to 200 mg of potassium iodide in anticipation of radioactive iodine exposure will largely prevent uptake by the thyroid gland, thereby reducing the irradiation dose delivered by more than 98%. The same amount given at intervals after131I absorption is progressively less effective, but still reduces uptake to less than half after a delay of three hours. The suppressive effect of one dose is of short duration and daily readministration of the agent is required for prolonged protection. The extent of uptake blockade may be estimated by measuring serum inorganic iodide; concentrations of greater than 10μg /100 cc correlate with marked uptake arrest. Toxicity is negligible for a single ingestion, as is required in this temporary measure for the reduction of the immediate thyroid irradiation hazard, but the drug must be avoided by those allergic to it.