The total number of PAH subjects enrolled will be up to 70. Subjects will undergo 1) standard clinical examinations including 6-minute walk distance and echocardiography; 2) cardiopulmonary exercise testing; 3) plasma profiling of inflammatory and neurohormonal markers; 4) gene expression profiling of peripheral blood mononuclear cells (PBMCs); and 5) high-resolution MRI-based determination of pulmonary vascular and RV structure and function. Safety and tolerability of spironolactone in PAH will be assessed with periodic monitoring for hyperkalemia and renal insufficiency as well as the incidence of drug discontinuation for untoward effects.