We describe a modified [from Anal. Chem. 36, 2099 (1964)] procedure for determining diazepam and its pharmacologically active metabolite, desmethyldiazepam, in plasma, with use of a nitrogen-sensitive detector in the gas-liquid chromatography. We used medazepam as the internal reference standard. Diazepam and desmethyldiazepam are converted to their respective benzophenones by mineral acid hydrolysis. With this procedure, as little as 100 muL of plasma can be used to determine the drug in concentrations as low as 10 microgram/L, accurately, reproducibly, and quickly. Within-run and between-run CVs for 100 microgram/L concentrations of the two compounds were 5 and 7%, respectively.