A method for the quantitative determination of pethidine (meperidine) and it metabolite norpethidine (normeperidine) in human hair by LC-MS-MS is presented. After acid incubation of 50mg hair powder with pethidine-d4 and norpethidine-d4 used as internal standard, a liquid-liquid extraction to pH 9.5 was achieved. Hair extracts were separated on an X-Terra MS C18 column using a gradient of acetonitrile and formate buffer pH 9.5 and were detected by tandem mass spectrometry in the positive electrospray ionisation mode. The ions transitions monitored in multiple reaction monitoring were m/z 248>174; m/z 252>178; m/z 234>160; m/z 238>164 for pethidine, pethidine-d4, norpethidine and norpethidine-d4, respectively. The recoveries were above 70% for both compounds. Linear responses were observed for pethidine concentrations ranging from 1 to 5000pg/mg (r²>0.999) and for norpethidine concentrations ranging from 2 to 5000pg/mg (r²>0.999), respectively. The quantification limits were estimated to be 2pg/mg for pethidine and 5pg/mg for norpethidine. The intraday and interday precision (measured by coefficient of variation) was less than 12%. The accuracy was observed ranging from 88% to 113% for both compounds. In the hair of an addict, concentrations were 0.85 to 2.32 and 0.29 to 3.25ng/mg for pethidine and norpethidine, respectively, in 4×3cm segments.