Propofol is an intravenous anaesthetic widely used for induction and maintenance of anaesthesia and for sedation. A sensitive and simple gas chromatography-mass spectrometry (GC-MS) method for the determination of 2,6-diisopropyl-1,4-quinol, the major hydroxylated propofol metabolite, is presented. Thymol was used as internal standard. Thymol and 2,6-diisopropyl-1,4-quinol are extracted under basic conditions. The extraction efficiencies for quinol metabolite was > 90 %. Standard curves were linear over the ranges of 50 to 2500 ng ml−1. This assay represents an alternative to existing methods of 2,6-diisopropyl-1,4-quinol analysis, which use HPLC. The assay was applied to the analysis of propofol metabolism in vitro by rat liver microsomes.