Melatonin protection against H2O2-induced lipid peroxidation in brain homogenates was measured in vitro. The level of malonaldehyde (MDA) plus 4-hydroxyalkenals (4-HDA) was assayed in brain homogenates as an index of induced membrane oxidative damage. Brain homogenates were co-incubated with H2O2 alone or in combination with melatonin. Brain MDA + 4-HDA increased after H2O2 (0.1-5 mM) with the effect being both concentration- and time-dependent. Melatonin (0.1-5 mM) protected against H2O2-induced lipid peroxidation of brain homogenates in a concentration-dependent manner.