A method using gas-liquid chromatography-electron-capture detection method is described for rapid, accurate determination of SAS 643 in plasma and urine. The drug is extracted from biological fluid with benzene and converted to the O, O'-bistrimethylsilyl derivative with bis(trimethylsilyl) trifluoroacetamide. The glucuronide form of the drug is extracted after hydrolysis with β-glucuronidase. Nimetazepam is used as internal standard. Moreover, some metabolites such as glucuronide and the N-1-dealkylated and N-1-yl-acetic products are identified. All compounds were confirmed by thinlayer chromatography, mass spectroscopy, and gas-liquid chromatography-mass spectroscopy by comparison with reference products.