Lipid peroxidation in vitro of carbon tetrachloride (CCL4) and ethylene dibromide (EDB) in microsomal supernatant fraction (MSF) of chick and rat livers was evaluated by measuring the malonaldehyde concentration after incubation with thiobarbituric acid. The additive peroxidation effect of CCl4 in rate liver NADPH-induced system, was confirmed. The addition of EDB to the system had no effect on lipid peroxidation. The addition of NADPH to MSF of chick liver caused a marked increase in peroxidation. Both CCl4 and EDB inhibited peroxidation in the above system. The different response of the two species to the CCl4 hepatotoxicity is discussed.