Specific deuterium labeling of methadone and use of gas chromatography-mass spectroscopy technique permits rapid and quanitative determination of the ratio of the labeled to unlabeled drug in body fluids. A trideuertiomethadone (methadone-d3) was shown to have exactly the same analgesic activity and toxicity in mice as methadone. The rates of absorption, distribution, and excretion of methadone-d3 and methadone were identical in rats. These observations suggest that methadone-d3 may be used as an in vivo marker for monitoring methadone intake of patients, and thus may improve the effectiveness of methadone treatment programs.