Abstract: AIMTo establish a RP-HPLC method for determination of diphenytriazol (DL111-IT) in rat hepatic microsomes. METHODSDL111-IT in rat hepatic microsomal incubates was extracted with chloroform, using diazepam as internal standard. The determination was performed on a Lichrospher ODS-C₁₈ reversed column (25 cm×0.46 cm ID) with mobile phase of methanol-pH 7.5 phosphate buffer (70∶30) at a flow-rate of 1.0 mL·min^-1. A UV-VIS detector was operated at 235 nm.RESULTSThe assay was linear from 1.01～101.0 μg·mL^-1 for DL111-IT. The limit of detection was 0.15 μg·mL^-1 (signal-to-noise ratio 3) and the limit of quantification was 1.01 μg·mL^-1 (RSD<10%, N=4). The method afforded average recoveries of (100.3±1.9)% (N=5), and intra-day and inter-day RSD were less than 5.0%(N=5). The method allowed study of the in vitro phase I metabolism of DL111-IT in rat liver microsomal incubates. The microsomes induced by β-naphthoflavone showed high enzymatic activity for DL111-IT phase I metabolism. CONCLUSIONThe method is simple, accurate and can be used to study the metabolism of DL111-IT in rat hepatic microsomes.