The effect of various doses of sodium perchlorate in several dose fractions on the extent and the time scale of displacement of radiopertechnetate, in dependence on application mode, was studied. An intravenous dose of 50 mg perchlorate was in respect of competitive suppression of organs actively concentrating pertechnetate as effective as intravenous 1000 mg ClO-4- simultaneously or 1000 mg orally 30 min before the injection of radiopertechnetate. An intravenous injection of perchlorate given later also produces a complete and immediately beginning depletion of pertechnetate already accumulated in the thyroid, within a period of 195 min after 99m-TcO-4-injection with a corresponding increase in blood levels. 20 mg result in incomplete depletion which becomes complete after a second additional dose. The intravenous application of perchlorate offers advantages in clinical use.