We tested the effect of melatonin on membrane fluidity in microsomes of a rat liver model in which lipid peroxidation was induced by the addition of FeCl3, ADP and NADPH. Membrane fluidity was monitored using fluorescence spectroscopy and lipid peroxidation was estimated by quantifying malonaldehyde (MDA)+4-hydroxyalkenals (4-HDA) concentrations following the induction of lipid peroxidation with and without pre-incubation with melatonin (1 microM-3 mM). Membrane rigidity increased during induced lipid peroxidation while melatonin reduced in a concentration-dependent manner both membrane rigidity and MDA+4-HDA generation. Melatonin's protective effect may relate to its known ability to scavenge free radicals and function as an antioxidant.