The disposition of methadone, its metabolites, and other illicit drugs of abuse was investigated in head hair samples collected from heroin users (N = 20) enrolled in an outpatient detoxification study. Hair samples were assayed for methadone, methadone primary metabolite (2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine, EDDP), methadone secondary metabolite (2-ethyl-5-methyl-3,3-diphenylpyrroline, EMDP), cocaine, phencyclidine, heroin, and 6-acetylmorphine. Hair samples were cut, washed, and incubated in methanol. The methanolic hair wash and incubation fractions were purified with solid-phase extraction and assayed by gas chromatography-mass spectrometry. Methadone, methadone metabolites, cocaine, and phencyclidine were assayed quantitatively, and other drugs were measured qualitatively. The number of positive results and the corresponding concentration ranges were as follows: methadone, 0-15.0 ng/mg (N = 18); EDDP, trace (N = 13); EMDP, trace (N = 1); cocaine, 0->40 ng/mg (N = 14); phencyclidine, 0-1.5 ng/mg (N = 2); heroin, positive (N = 3); and 6-acetylmorphine, positive (N = 4). These data suggest that testing hair for methadone, methadone metabolites, and other illicit drugs of abuse may be useful to drug-treatment specialists as a means of verifying drug-use history, monitoring compliance, and providing a broad measure of drug exposure.