The aim was to investigate the metabolic activation potential of a pentafluorophenylethylamine derivative (compound I) in vitro in the rat and to identify the cytochrome P450 (CYP) enzymes that catalyse these metabolic activation processes. Reduced glutathione (GSH) was fortified in rat hepatocytes and liver microsomes to trap possible reactive intermediates. Four glutathione conjugates (M1-4) were identified by LC-MS(n) following incubation of compound I in GSH-enriched rat hepatocytes and liver microsomes. Three of these conjugates (M2-4) have not been reported previously for pentafluorophenyl derivatives. Elemental composition analysis of these conjugates was obtained using high-resolution quadrupole time-of-flight mass spectrometry. The formation of GSH conjugate M1 was rationalized as a direct nucleophilic replacement of fluoride by glutathione, whereas the formation of the GSH conjugates M2-4 was proposed to occur by NADPH-dependent metabolic activation of the pentafluorophenyl ring via arene oxide, quinone and/or quinoneimine reactive intermediates. Formation of these conjugates was enhanced three- to five-fold in liver microsomes obtained from phenobarbital- and dexamethasone-treated rats. In incubations with pooled rat liver microsomes and recombinant rat CYP3A1 and CYP3A2, troleandomycin (TAO) reduced the formation of GSH conjugates M2-4 by 80-90%, but it had no effect on the formation of M1. Incubation of compound I with rat supersomes indicated that only CYP3A1 and CYP3A2 were capable of mediating these metabolic activation processes.