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 microliter of toluene from 200-500 microliter of plasma, clonazepam is quantitated on a SP-2250 column with desmethyldiazepam 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 1 ng/ml of plasma, which is sufficiently sensitive for both therapeutic monitoring of plasma levels and pharmacokinetic evaluation in patients and experimental animals.