Background: Marjoram is an herbal plant with different medicinal effects. This study evaluates its impact on enzymatic antioxidant status, growth performance, intestinal mucosa morphology, and pulmonary hypertensive response in cold-induced pulmonary hypertensive chickens. 
Materials and Methods: Broiler chicks were reared for 35 days under cold stress and treated with 0.05% vitamin C (positive control) and 0 (control), 0.1%, or 0.2% marjoram extracts. Serum malondialdehyde (MDA) and activity of catalase (CAT), glutathione peroxidase (GPX), and superoxide dismutase (SOD) were assayed on day 35. Meanwhile, gene expression of these enzymes was evaluated in the duodenum.
Results: The right ventricle to total ventricles (RV:TV) ratio was lower in all treatments of chickens than control (P<0.05). The feed conversion ratio was only decreased in the chickens fed marjoram-0.2%. Lipid peroxidation was reduced in all groups, while the CAT activity was increased in the marjoram-0.2% group compared to the control (P<0.05). In the lung, SOD, CAT, and  GPX transcripts were decreased in the marjoram-0.2% group compared to the control (P<0.05). In the right ventricle of the heart, SOD and CAT transcripts were increased in the marjoram-0.2% group compared to other groups of chickens, whereas GPX transcript was decreased (P<0.05). In comparison to the control, the chickens fed vitamin C and marjoram had longer duodenal villus and more surface area, and their villus lamina propria was thicker (P<0.05). 
Conclusion: Supplementation of marjoram could modulate pulmonary hypertensive response and ameliorate intestinal morphology through its antioxidant effects.