Twenty-one opiate-dependent subjects were inducted into methadone maintenance treatment (MMT) in a closed metabolic ward. A daily dose of 30 mg of d,l-methadone was given for 10 to 24 days followed by 60 mg Iday for another 10 to 24 days. Analysis of plasma levels at 4-day intervals showed accumulation to a peak followed by a decrease to a lower level, indicative of development of dispositional tolerance. The outcome of treatment was assessed after 21 to 43 mo (median, 33 mo). The best record of rehabilitation was obtained in subjects discharged with steady-state plasma concentrations above 200 ng/ml. Lower levels of plasma methadone were associated with higher frequency of urines containing illicit drugs and poorer psychosocial rehabilitation. This study indicates that a pharmacokinetically optimized dosage regimen would be useful in increasing the therapeutic effectiveness of MMT.