This is a prospective, multi-center, open-label, randomized, Phase IV exploratory study comparing safety, tolerability, pharmacokinetics, and effectiveness of ACT-385781A and Flolan (epoprostenol sodium) in patients with pulmonary arterial hypertension who are naïve to injectable prostanoid treatment and in need of such treatment. Approximately 30 patients from 8 U.S. clinical sites will be randomized to receive either ACT-385781A or Flolan (2:1 respectively) for 28 days of treatment.