The hepatotoxicity of the anticonvulsant drug valproic acid (VPA) is most likely associated with the bioactivation of its metabolite 2-propylpent-4-enoic acid (4-ene VPA), which is known to induce hepatic microvesicular steatosis in rats. This paper presents an on-line liquid chromatographic/tandem mass spectrometric (LC/MS/MS) identification of new glutathione (GSH)-related conjugates of the reactive metabolites of 4-ene VPA. Bile samples collected from male Sprague-Dawley rats dosed intraperitoneally with 4-ene VPA or its [2H7]-analogue (100 mg kg-1) were injected on to an ODS column interfaced to a LC/MS/MS instrument using electrospray ionization. LC was developed such that no overlapping of peaks occurred among those metabolites which may potentially produce common fragment ions of interest. Subsequent comparison of LC retention times and MS/MS full fragment ion spectra generated for putative metabolites with that of authentic reference compounds made available by chemical synthesis confirmed the presence of the GSH, cysteinylglycine, cysteine and N-acetylcysteine (NAC) conjugates of 2-(2'-carboxypentanyl)oxirane (4,5-epoxy VPA) and (E)-2-propylpenta-2,4-dienoic acid ((E)-2,4-diene VPA), respectively. Quantitatively, the biliary thiol conjugates accounted for 5% of the dose. This observation is novel for 4-ene VPA metabolism in terms of the degradation of GSH conjugates to the corresponding mercapturic acids possibly occurring within the liver as opposed to an inter-organ process which involves the kidney. In addition, the GSH- and NAC-glucuronide di-conjugates of (E)-2,4-diene VPA were also identified as the biliary metabolites with the GSH-glucuronide di-conjugate being 10% of the corresponding mono-GSH conjugate. Taken together, these data clearly indicate that reactive metabolites of VPA can react with hepatic GSH via several different metabolic pathways and may subsequently produce depletion of GSH that leads to toxic consequences.