Background: Melatonin, an endogenously produced indoleamine, is a highly effective antioxidant and free radical scavenger. This study investigates the potential antioxidant effects of melatonin in birds exposed to conditions promoting pulmonary hypertension syndrome (PHS) under cold stress. 
Materials and Methods: Two groups of birds were offered different concentrations of melatonin (0.2% and 0.4%), while a control group received no melatonin treatment. Serum and cardiac tissue samples were collected to evaluate glutathione peroxidase (GPX) and superoxide dismutase 1 (SOD1) activities and the relative expression of GPX and SOD1 genes. 
Results: The results showed a significant decrease in the right ventricular to total ventricular weight ratio in the melatonin 0.4% supplemented group compared to the control. Melatonin supplementation at 0.2% and 0.4% levels led to lower levels of malondialdehyde (MDA) in the serum and heart compared to the control group, indicating reduced lipid peroxidation. Both melatonin groups exhibited increased serum/cardiac (GPX) activities compared to the control group; however, the serum SOD1 activity was only increased in the melatonin 0.4% group of birds compared to the control group. Furthermore, the melatonin-0.2% and -0.4% groups displayed decreased relative gene expression of GPX and SOD1 compared to the control group. 
Conclusion: Melatonin, especially with a dose of 0.4%, when used as an antioxidant agent, can be beneficial in reducing the severity of PHS and heart right ventricular failure in birds.