A rapid and simple analytical method is described for the determination of clonazepam in plasma using electron capture gas chromatography. Following a single microextraction with 100 μl of toluene from 200–500 μl of plasma, clonazepam is quantitated on a SP-2250 column with desmethyldi-azepam as internal standard. There is no interference from endogenous substrates, clonazepam metabolites, or other commonly used antiepileptics. The lowest detectable clonazepam concentration by this method is about l ng/ml of plasma, which is sufficiently sensitive for both therapeutic monitoring of plasma levels and pharmacokinetic evaluation in patients and experimental animals.