Angiotensin-converting enzyme 2 (ACE2), an ACE homolog, hydrolyzes angiotensin II and opposes its actions, and plays a protective role in the pathogenesis of pulmonary arterial hypertension (PAH). However, the underlying mechanisms involved in the effect of ACE2 on PAH are still uncertain. In this study, we observed the effects of ACE2 activation on endothelial dysfunction and vascular remodeling in the development of severe PAH in rats. Severe PAH was induced by monocrotaline injection 1 week following left pneumonectomy, and ACE2 was activated by continuous injection of resorcinolnaphthalein. The PAH-related hemodynamics, pathological changes, and endothelium-dependent vasorelaxation were examined to assess the effects of ACE2 activation. In addition, the changes of the main components of the renin-angiotensin system were identified by ELISA or Western blotting. Severe PAH was established at 3 weeks and was characterized by high pulmonary arterial pressure (45 mmHg), significant right ventricular hypertrophy, neointimal occlusive lesions, and impaired endothelium-dependent relaxation in pulmonary arteries. Coadministration of resorcinolnaphthalein reduced pulmonary arterial pressure, right ventricular hypertrophy, and neointimal formation and shifted the endothelial-dependent responses toward values measured in normal rats. Theses changes were associated with an increase in ACE2 and angiotensin-(1-7) levels and a decrease in ACE and angiotensin II levels, in addition to a decrease in the ACE/ACE2 ratio and the angiotensin II/angiotensin-(1-7) ratio. The beneficial effects of resorcinolnaphthalein were abolished by A-779. These findings suggested that ACE2 activation by resorcinolnaphthalein improved endothelial function and suppressed neointimal formation in the prevention of severe PAH by the mechanism of mediating the levels of the components of the renin-angiotensin system.