Ten volunteer patients participating in a methadone hydrochloride maintenance program were assigned randomly to experimental (five patients) or control (five patients) groups. These patients had been in treatment for several months and had been stabilized (had no change of dose for at least three weeks) with methadone prior to inclusion in the study. Patients in the experimental group were given l-methadyl acetate (l-α acetylmethadol) on weekends and methadone on weekdays. Patients in the control group were given methadone each day. Clinic attendance, requests for change in medication, and scores on an opiate-withdrawal test instrument did not reveal differences between groups. Clinical observers blind to the experiment were unable to discriminate experimental and control patients. No untoward reactions were observed in either group. These preliminary observations suggest that l-methadyl acetate can be interchanged repeatedly with methadone without difficulty.