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Background: Despite the available treatments, pulmonary arterial hypertension (PAH) prognosis is poor., Objectives: We aimed to investigate the effects of the alamandine (ALA), melatonin (MEL), and ALA + MEL in PAH., Methods: The rats were randomly divided into Control (n = 10), monocrotaline (MCT) (n = 12), ALA (n = 12), MEL (n = 12), and ALA + MEL (n = 12) groups. PAH was induced by MCT. The ALA, MEL, and ALA + MEL groups received 50 μg/kg/day ALA, 10 mg/kg/day MEL, and ALA + MEL, respectively, for 35 days. Echocardiographic and hemodynamic measurements and tissue analyses (morphometric, histopathological, ELISA, and western blot) were performed., Results: Monotherapies, especially MEL, reduced the right ventricular (RV) systolic pressure. Only MEL increased the pulmonary artery acceleration time. MCT increased the RV/left ventricle (LV) + interventricular septum (IVS) ratio. While ALA and ALA + MEL slightly decreased the RV/(LV + IVS), MEL significantly restored it. MCT increased the tunica intima-media (TIM) thickness, PCNA and α-SMA of pulmonary arterioles, histopathological score (HS) (inflammatory infiltration etc.) of the lung, and RV. All treatments reduced the TIM thickness (especially MEL), PCNA, and α-SMA. All treatments significantly decreased the HS of the lung; however, MEL and ALA + MEL produced greater benefits. All treatments attenuated the HS of RV. MCT caused a significant increase in lung lysyl oxidase (LOX) activity. All treatments restored the LOX; however, MEL and ALA + MEL provided greater improvement. While lung Nrf-2 was increased in MCT-treated rats, MEL reduced it., Conclusion: ALA, MEL, and ALA + MEL attenuate PAH and protect RV via antiproliferative, anti-remodeling, antihypertrophic, anti-inflammatory, and free radical scavenging (only MEL) capabilities. Overall, MEL produced the best outcomes., (© 2024 Société Française de Pharmacologie et de Thérapeutique. Published by John Wiley & Sons Ltd.)