Stimulation of MAS oncogene receptor (MAS) or angiotensin (Ang) receptor type 2 (AT2) may be novel therapeutic options for neonatal chronic lung disease (CLD) by counterbalancing the adverse effects of the potent vasoconstrictor angiotensin II, consisting of arterial hypertension (PAH)-induced right ventricular hypertrophy (RVH) and pulmonary inflammation. We determined the cardiopulmonary effects in neonatal rats with CLD of daily treatment during continuous exposure to 100% oxygen for 10 days with specific ligands for MAS (cyclic Ang-(1-7); 10-50 µg/kg/day) and AT2 (dKcAng-(1-7); 5-20 µg/kg/day). Parameters investigated included lung and heart histopathology, fibrin deposition, vascular leakage and differential mRNA expression in the lungs of key genes involved in the renin-angiotensin system, inflammation, coagulation and alveolar development. We investigated the role of nitric oxide synthase (NOS) inhibition with N-nitro-L-arginine methyl ester (L-NAME, 25 mg/kg/day) during AT2 agonist treatment. Prophylactic treatment with agonists for MAS or AT2 for 10 days diminished cardiopulmonary injury by reducing alveolar septum thickness and medial wall thickness of small arterioles, and preventing RVH. Both agonists attenuated the pulmonary influx of inflammatory cells, including macrophages (via AT2) and neutrophils (via MAS), but did not reduce alveolar enlargement and vascular alveolar leakage. The AT2 agonist attenuated hyperoxia-induced fibrin deposition. In conclusion, stimulation of MAS or AT2 attenuates cardiopulmonary injury by reducing pulmonary inflammation and preventing PAH-induced RVH, but does not affect alveolar and vascular development in neonatal rats with experimental CLD. The beneficial effects of AT2 activation on experimental CLD were mediated via a NOS-independent mechanism.