An HPLC method with electrospray ionization and ion-trap mass spectral detection (LC-ESI/MSD Ion Trap SL) for determining nibentan metabolites in human blood plasma is described.The proposed method provides simultaneous determination of nibentan and its metabolites in blood plasma.Three metabolites, hydroxylated, de-ethylated, and de-ethylated-hydroxylated, were found in human blood plasma after a single i.v. administration of nibentan (0.125 mg/kg, 1%).De-ethylated nibentan was the longest lived metabolite.The last 2 metabolites were found in human blood plasma for the first time.The results were verified and all metabolites were identified using LC-MS and LC-MS/MS.The major pharmacokinetic parameters were calculated