SummaryThiouracil treatment has been given to 18 hyporthyrotoxic patients and aminothiazol to 7 others.The results, difficulties and dangers of these new anti-thyroid treatments aro reported. No severe toxic reactions have been encountered although half of the patients treated with thiouracil showed leucopenia. In two cases this blood-marrow reaction was severe and, as shown by the myelogramm, on the verge of agranulocytosis.With aminothiazol, no interference with white blood cell formation was observed. Out of the 7 patients treated, 2 showed vomiting and 1 had a marked and relapsing urticaria.