It has been proposed that lipid peroxidation (LP) might be a modulator of cell division, influencing initiation and cessation of mitosis in regenerating liver. However, the understanding of the participating role of this event in the onset of liver proliferation has been hampered by the fact that both higher or lower LP have been reported after two-thirds partial hepatectomy (PH). Therefore, the present study deals with the extent of LP in the main subcellular fractions from rat liver at early stages of regeneration, induced by either PH of 70% or acute CCl4 administration. Our results, using several methods to monitor LP, indicate a differential effect in the peroxidative pattern of specific subcellular fractions from regenerating liver after 24 hours of PH a decrease in microsomes and an increase confined to plasma membrane and cytosolic fractions, peaking after 24 hours of PH. In CCl4-treated rats, higher LP was also noted in plasma membrane and cytosol, being maximal at the replicative stage in this experimental model (48 hours). In addition, increased LP was found in microsomal and nuclear fractions, declining before the 48 hours. In hepatectomized rats, changes in LP seem to be an organ-specific event and related to only PHs capable of triggering a synchronized proliferative response, namely above 40%. These results show that LP, promoted by PH and CCl4 administration, is qualitatively distinct among subcellular fractions and may indeed be a normal cell event of physiological importance in the regenerating liver.