AFTER the invasion of Germany by the Allied armies, a Department of Commerce intelligence team1 which investigated the German pharmaceutical industry found that the chemists of the I. G. Farbenindustrie had developed a potent synthetic analgesic drug, 6-dimethylamino-4-4-diphenyl-3-hepatone, which bore the serial number 10820.2 Investigation of the pharmacology of the drug in the United States showed that many of its actions resembled closely the actions of morphine in both animals3 and man.4 The question of the liability of addiction to the drug was therefore immediately raised. It is the purpose of this paper to present the results of studies on the liability of addiction to methadon which were carried out on former morphine addicts who volunteered for the experiments. In these studies particular attention was paid to the qualities which Himmelsbach5 regards as characteristic of addiction to a drug: tolerance, physical dependence and habituation (psychologic