We propose to assess the evolution of both circulating and pulmonary venous EMPs in patients with PH. Assessments will be made before and after initiation of Endothelin-1 (ET-1) Receptor blocker therapy, and correlated to their patterns with the changes in mean PAP, the 6 Minutes Walking Distance test, and circulating Endothelin-1 values. Measurements of the endothelial microparticle circulating levels (assessed by flow cytometry methods) will be made before, 1 month and 3 months after initiation of therapy.