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A sensitive method of determination of ketamine and norketamine by micro-liquid chromatography/mass spectrometry was developed as part of a clinical trial in a pediatric population. Compounds were extracted from 100 mu l plasma samples using solid-phase extraction on Oasis (R) MCX cartridges. Separation was achieved on a C18 micro-column with a mobile phase composed of formic acid 0.1% and acetonitrile (90/10, v/v). Detection of ketamine, norketamine and their internal standard norketamine D4 was performed using selected ion monitoring at m/z 238.2,224.2 and 228.2, respectively. Under these conditions, no loss of signal due to matrix effect was found and time of analysis did not exceed 10 min. Extracted calibration curves were linear from 5 to 500 ng/ml for each analyte, with correlation coefficients over 0.999. Intra- and inter-day validation studies showed mean recoveries between 98.1 and 101.7% and a relative standard deviation below 1.9%. Extraction recoveries ranged from 84.8 to 89.8% for both ketamine and norketamine. Limit of quantification was 4 ng/ml for each analyte. (c) 2008 Published by Elsevier B.V.