AIM: To study the protective effect of a natural antioxi- dant, melatonin, against multistress condition induced lipid peroxidation via determination of gastric damage and plasma malondialdehyde (MDA) level by high perfor- mance liquid chromatography in rats. METHODS: We compared indomethacin-induced gas- tric damage and MDA plasma level in three groups of rats: unoperated, bile duct ligated and sham-operated and evaluated the role of the melatonin on gastric dam- age and plasma MDA level. Indomethacin and melatonin were injected intraperitoneally in doses of 50 mg/kg and 20 mg/kg, respectively. Animals were killed 4 h after in- domethacin injection. RESULTS: Indomethacin induced more severe gastric damage and plasma MDA level in bile duct ligated ani- mals was significantly higher (3.1 ± 0.04 μmol/L) than sham (2.8 ± 0.04 μmol/L) and unoperated animals (1.4 ± 0.08 μmol/L). Pretreatment with melatonin reduced indomethacin-induced gastric damage and plasma MDA level. CONCLUSION: Considering the results of this study, we suggest that in multistress conditions the intensity of gastric damage and the plasma MDA level are great and melatonin reduces the negative effect of lipid peroxida- tion and cell damage by oxidative stress in multistress conditions due to its antioxidizing activity. AIM: To study the protective effect of a natural antioxi- dant, melatonin, against multistress condition induced lipid peroxidation via determination of gastric damage and plasma malondialdehyde (MDA) level by high perfor mance liquid chromatography in rats. METHODS: We compared indomethacin- induced gas-tric damage and MDA plasma level in three groups of rats: unoperated, bile duct ligated and sham-operated and evaluated the role of the melatonin on gastric dam- age and plasma MDA level. Indomethacin and melatonin were injected intraperitoneally in doses of RESULTS: Indomethacin induced more severe gastric damage and plasma MDA level in bile duct ligated ani- mals was significantly higher (3.1 ± 0.04 μmol / L) than sham (2.8 ± 0.04 μmol / L) and unoperated animals (1.4 ± 0.08 μmol / L). Pretreatment with melatonin reduced indomethacin-induced gastric damage and plasma MDA level. CONCLUSION: C onsidering the results of this study, we suggest that in multistress conditions the intensity of gastric damage and the plasma MDA level are great and melatonin reduces the negative effect of lipid peroxida- tion and cell damage by oxidative stress in multistress conditions due to its antioxidizing activity .