N-Desmethyldiazepam in plasma has been determined by gas chromatography with electron capture detection, using N-desmethyltetrazepam as internal standard and a column filled Poly-A 103 on Gas-Chrom Q. No prior transformation of benzodiazepines is necessary. The sensitivity is ca.1 ng/ml and the recovery is greater than or equal to 70%. The method is particularly suitable for the pharmacokinetic study of dipotassium clorazepate: the amount of unchanged clorazepate may be estimated from the difference between the amount of N-desmethyldiazepam present initially and the total amount of N-desmethyldiazepam obtained after acidic treatment of the sample; the method is also useful for therapeutic control and for analytical toxicology.