A specific procedure for the analysis of clobazam [a tranquilizer] and N-demethylclobazam in plasma is described. Reversed-phase liquid chromatography was performed on a Radial-pak cartridge using a mixture of 45% acetonitrile and 55% buffer solution (pH 7); the UV detector was set at 254 nm. The method used diazepam as internal standard and diethylether as extraction solvent. The calibration curves were linear between 50-500 ng/ml for clobazam and between 100-1000 ng/ml for N-demethylclobazam. The day-to-day precision of the procedure at clobazam plasma concentrations of 50, 250 and 500 ng/ml generated coefficients of variation of 2.2, 6.6 and 11.3%, respectively. No interference occurred in plasma from patients treated with various drugs. The method was used to study the pharmacokinetics of clobazam and N-demethylclobazam in patients receiving oral clobazam.