A rapid method is described for the electron-capture gas chromatographic determination of clonazepam in plasma or serum using methyl-clonazepam as an internal standard. The analysis is performed isothermally on the silicone stationary phase SP-2510DA (Supelco). With this liquid phase, gas chromatographic properties are comparable to methods involving acid hydrolysis or derivatisation. A short pre-column containing another phase is added to enhance resolution. The method involves a single extraction, requires 100 microliter of sample and has a detection limit of 3 nmol/l. Response is linear at concentrations from 5--900 nmol/l and thus clonazepam analysis both during therapy and after overdosage is possible. Plasma and serum clonazepam levels are interchangeable.