Pulmonary arterial hypertension (PAH) is an orphan disease characterized by pulmonary artery hypertrophy, and resulting vascular remodeling of involved vessels, often leading to right heart failure. Accumulating evidence from vascular biology, animal models, and therapeutic drug trials suggests significant contributions of the neurohormonal milieu to the disease process, morbidity, and mortality. The renin-angiotensin-aldosterone system (RAAS) is an important neurohormonal pathway that induces collagen synthesis in the myocardium and systemic vasculature. There is paucity of data regarding the contribution of RAAS in the pathogenesis of PAH and the effects of aldosterone blockade in the amelioration of PAH. Thus, the overall goal of this proposal is to investigate the contribution of RAAS to the pathogenesis of PAH, and to explore the effects of an aldosterone blocker, spironolactone, in PAH.