A simple and sensitive method for quantifying 2-chlorophenyl-N-desmethyldiazepam (CND), which is the major metabolite in plasma after administration of mexazolam, has been developed using gasliquid chromatography equipped with electron capture detector (GC-ECD). After extraction with benzene from 2ml of plasma specimens, CND could be quantitated to a lower limit of 0.1 ng/ml. A single oral dose of mexazolam was administrated to ten healthy subjects in order to reveal the pharmacokinetics of mexazolam. The time course of CND in plasma followed the equation of a twocompartment model with an appearance lag time. The mean peak plasma level, 6.80±0.68 ng/ml, was attained at 2.01±0.55 hours after administration of mexazolam, and the apparent elimination half life was 76.4±6.35 hours. The speculated plasma CND profiles reached approximately 90 % of the steady state levels on the 11th day after the initiation of repetitive doses of mexazolam. The calculated average plasma level at steady state, Casssv following the hypothetical dosage schedule (3 mg of mexazolam administrated per day), was about 65 ng/ml.