Plasma drug concentrations were determined and behavioral observations were made in 11 subjects who transferred from methadone administered every 24 hr to /- α -acetylmethadol (LAAM) administered every 48 hr Plasma concentrations of methadone, LAAM and its metabolltes, noracetylmethadol [N-LAAM] and dinoracetylmethadol [DN-LAAM] were determined 24 hr after the last dose of methadone and 48 hr after each dose of LAAM for the first 10 LAAM doses (19 days). Methadone could be detected in plasma for up to 5 days after the last dose. LAAM plasma levels were low [approximately 35 ng/ml] and did not accumulate with chronic administration. Plasma levels of N-LAAM and DN-LAAM increased 3- and 4-fold respectively with time and did not reach steady state until after day 11 (dose 6). Two of the 11 subjects experienced overdosing symptoms during the study, and their plasma levels of N-LAAM and DN-LAAM were significantly higher than those found in the other subjects.