Prostacyclin (PGI2) is the main product of arachidonic acid in all vascular endothelium. It is a potent vasodilator in all vascular beds. In addition, it is a potent endogenous inhibitor of platelet aggregation and smooth muscle growth. The prostacyclin receptor (IP) is located on a variety of cell types, enabling prostacyclin to exert a range of biologic actions by means of raising intracellular levels of cAMP through activation of adenylate cyclase. The stable, freeze-dried salt preparation of prostacyclin is known as epoprostenol and is available for IV administration under the brand name Flolan (Glaxo Wellcome, Research Triangle Park, N.C.). The first randomized clinical trial in PPH showed that epoprostenol improved quality of life, hemodynamics, exercise tolerance, and survival over a 12-week period. Epoprostenol has become the standard of care for patients with advanced PPH. Recently, chronic intravenous epoprostenol has been shown to be an effective therapy to improve long-term quality of life and survival in patients with PPH. Epoprostenol also improves hemodynamics, exercise capacity, and quality of life (but not survival) in PAH associated with CHD and connective tissue disease.