Abstract: A simple rapid and sensitive HPLC method for simulaneous determination of N-demethyldiazepam and its metabolite oxazepam i n both human and rat plasma was described.Areversed phase C₁₈ column equipped with a 240 nm detector were selected. Diazepam was used as internal standard.The mobile phase consisted of acetonitrile—0. 01 mol·L^-1 sodium acetate buffer(pH3.8,33.3 : 66.6v/v). The retention time for N-demethyldiazepm, oxazepam and internalstandard were 4. 85 min,2. 8 min and 8. 5 min, reapectively. The lower detection limits foroxazepm and N- demethyldiazepam were 10 ng ml-1 and 7 ng· ml^-1 ,respectively. The precisions(RSD%,n=5 ) of within-day and day-to-day were less than 6%and 1 0%and their absoluterecoveries were 74%and 86%, respectively. The pharmacokinetic characteristics of N-demethyldiazepam in Sprague-Dawley rats after a single oral dose of 2.5mg· kg^-1 was investigated by thismethod, The concentration-time curve was fitted to a two-compartment open model.Its main pharma-cokinetic parameters were:T_max=0. 24h,C_max=1211.78ng·ml^-1,T_1/2β=2.84h, AUC=1264ng·ml^-1·h^-1.