A sensitive and highly specific gas-liquid chromatographic method is described for the determination of benzodiazepine esters. The method involves the extraction of pivoxazepam and 2'-chloropivoxazepam from blood and urine samples and the hydrolysis of both drugs with strong acid to the corresponding 2-amino-5-chlorobenzophenone (ACB) and 2-amino-5,2'-dichlorobenzophenone (ACDB). Recoveries of 97-98% have been obtained and the trifluoroacetic (TFA) derivatives of both benzophenones (ACB-TFA and ACDB-TFA) can be chromatographed on OV-17, in a short time, with good electron-capture detector (ECD) responses. The mass spectra of these derivatives provide abundant molecular ions suitable for mass spectrometric detection by multiple ion detection (MID), with a considerable improvement in specificity compared with the ECD responses. The detection limits in blood specimens have been established at 5-10 ng/ml by ECD and 0.5-0.8 ng/ml by MID. The method has been applied to the study of blood levels attained after i.v. and oral doses in rats and in man.