Modification of proteins and other biomolecules by aldehydes generated as secondary lipid peroxidation products is believed to play a role in a large number of biological processes, including aging, atherosclerosis and Alzheimer. Covalent binding of a,b-unsaturated aldehydes to aminoacids and proteins has being extensively demonstrated. The purpose of this study was to investigated the reaction of trans,trans-2,4-decadienal (DDE), a lipid peroxidation product, with cytochrome c in vitro and in rat liver mitochondria. Reaction of cytochrome c with DDE resulted in increases in molecular weight consistent with the formation of aldehyde Schiff base adducts as determined by MALDI-TOF spectrometry experiments. Adduct formation is favored under alkaline pH.  Moreover, a marked decrease in adducts stability was observed under acid pH.  The digestion of native and modified cytochrome c with both trypsin and chymotrypsin showed that fragments rich in basic aminoacid residues are more liable to be modified by DDE. Studies with isolated rat liver mitochondria showed that DDE increases the rate of oxygen consumption suggesting a partial uncoupling. Experiments performed in presence of GTP indicate that UCP proteins are not involved in the DDE-induced mitochondrial uncoupling. The covalent modification of cytochrome c by DDE could play a significant role in mitochondrial dysfunction associated with oxidative stress.