Methadone is narcotic analgesic which is used as analgesic and drug in substitution therapy of opiate addicts. The aim of this study was to develop method of determination of methadone (MD) and its main metabolite – 2-ethylidieno-1,5-di¬methylo-3,3-diphenylopyrolidine (EDDP). Methadone and EDDP was isolated from serum in a base medium (pH = 9) by liquid-liquid extraction using hexane. For the analysis of extracts, the high performance liquid chromatography and mass spectrometry (HPLC/MS) method was used in the electrospray ionisation mode (ESI), where the following were used as mobile phases: A – water with 0.1% addition of formic acid and B – acetonitrile with 5% addition of A phase on Finnigan-MAT apparatus. Chromatographic separation was carried out on a LiChroCART^® column. The method developed was characterised by linearity of concentration ranging from 3 to 500 ng ml^–1 and, for EDDP from 5 to 500 ng ml–1. Limit of detection (LOD) equalled 1 and 3 ng ml^–1 for methadone and EDDP, respectively. Limit of quantitation (LOQ) was 3 and 5 for MD and EDDP respectively. The recovery fluctuated between 84.3 and 94.3% for methadone and 90.6–98.7% for EDDP. The intra-day precision was below 3% whereas inter-day precision was below 7%. A good correlation between concentration of methadone and dose was shown. The method developed has been introduced to routine methadone monitoring of opiates addicts.