In recent years, remarkable advances in sensitive analytical techniques have enabled the analysis of drugs in unconcentional samples, such as sweat. In a study conducted during a methadone maintenance program, PharmChek sweat patches were applied to 20 subjects. The subjects were orally administered methadone in 1 dosage/day, and doses ranged from 80 to 100 mg. The sweat patch was applied 10 minutes before administration and removed 72 hours later just before a new administration of methadone. The absorbent pad was stored at -20°C until analysis in plastic tubes. Methadone was extracted in 5 ml methanol in presence of 200 ng of racemic methadone-d3, used as internal standard. After a 30-minute agitation, the methanol solution was evaporated to dryness. Enantioselective separation of methadone was obtained using an alpha-1-acid glycoprotein column (100 × 4 mm ID) and liquid chromatography/ion spray-mass spectrometry. In all 20 specimens obtained from subjects under racemic methadone treatment, R- (the active form) and S- enantiomers of methadone were identified with the following concentrations: 26 to 1118 ng/patch for R-methadone and 28 to 1114 ng/patch for S-methadone. The ratio between R- and S-methadone was in the range of 0.72 to 2.66 and was higher than 1.00 in 15 samples. No correlation between the doses of methadone administered and the concentrations of methadone in sweat was observed.