Lipid peroxidation in cellular membranes leads to the formation of toxic aldehydes. One product provided with particular reactivity has been identified as 4-hydroxynonenal and thoroughly studied as one of the possible mediators of the cellular injury induced by pro-oxidants. In the present study we have searched for the presence of 4-hydroxynonenal and other lipid peroxidation products in the liver of bromobenzene-poisoned mice, since under this experimental condition the level of lipid peroxidation is much greater than in the case of CCl4 or BrCCl3 hepatotoxicity. 4-Hydroxynonenal was looked for in liver extracts as either free aldehyde or its 2,4-dinitrophenylhydrazone derivative. In both cases, by means of thin-layer chromatography (TLC) and high-pressure liquid chromatography, a well resolved peak corresponding to the respective standards (free aldehyde or 2,4-dinitrophenylhydrazone derivative) was obtained. Total carbonyls present in the liver of intoxicated animals were detected as 2,4-dinitrophenylhydrazone derivatives. The hydrazones were pre-separated by TLC into three fractions according to different polarity (polar, non-polar, fraction I, and non-polar, fraction II). The amounts of carbonyls present in each fraction were determined by ultraviolet-visible spectroscopy. 'Non-polar carbonyls, fraction II' were further fractionated by TLC. The fraction containing alkanals and alk-2-enals was analyzed by high-pressure liquid chromatography and several aldehydes were identified. In addition, protein bound carbonyls were determined in the liver of bromobenzene-treated mice. The biological implications of the finding of 4-hydroxynonenal and other carbonyls in vivo in an experimental model of hepatotoxicity are discussed.