A stable blockade against the narcotic effects of heroin can be maintained by a single daily oral dose of methadone. Blockade is established by gradual increase in dosage to a stabilization level; thereafter the dose of methadone remains constant. With a blockade produced by a maintenance dose of 100 mg methadone, patients become refractory to the euphoric action of 80 mg or more of heroin[long dash]an amount equivalent to the drug contained in several illegal "bags" in New York. This procedure is being tested as an adjunct in rehabilitation of "mainline" addicts who had repeatedly relapsed after previous attempts at withdrawal and psychotherapy. With the maintenance treatment, the patients have lost their craving for heroin. No patient has become readdicted to heroin. A majority of the patients are now steadily employed. The therapeutic trial, started 2 1/4 years ago, is continuing.