Bacterial lipopolysaccharide (LPS) causes lipid peroxidation (LPO). We have found that LPS induces LPO in tissue homogenates in vitro in a concentration-dependent manner, the concentration of 400 μg/ml demonstrating the most efficient lipid damaging effect. Antioxidant properties of melatonin are unquestionable and have been proved both in vivo and in vitro. It has been demonstrated that also melatonin metabolites and derivatives inhibit oxidative stress. The aim of our study was to compare the effects of melatonin (MEL) and indole compound: 6-methoxytryptophol, on LPS-induced LPO in vitro. Malondialdehyde (MDA) plus 4-hydroxyalkenal (4-HDA) concentrations were measured as the indicators of induced membrane peroxidative damage in brain, liver and kidney homogenates. Both melatonin and 6-methoxytryptophol were used at increasing concentrations, starting from 0.01-5 mM, together with LPS at one concentration of 400 μg/ml. In all the examined tissues, LPS stimulated LPO, while both melatonin and 6-methoxytryptophol released LPS-stimulated LPO. Furthermore, the capacity of 6-methoxytryptophol reducing LPO was higher than that of melatonin. The results of the reported study clearly indicate that 6-methoxytryptophol is a much stronger antioxidant in vitro than melatonin in terms of reducing oxidative damage to lipid membranes. However, it remains still unclear how the features relate to in vivo circumstances.