The antioxidant behaviour of melatonin and structural analogues during lipid peroxidation depends not only on their functional groups but also on the assay system

► 5MTP and MLT did not show antioxidant effect on any of the systems studied. ► NAS and 5HO-TRP had antioxidant effect in triglycerides in chloroform whereas in liposomes in water had not. ► Differences between the antioxidant effect of indoleamines in both systems may be explained by the Polar Paradox theory.