Subjects who are already receiving FLOLAN (epoprostenol sodium) for the treatment of PAH and have been on a stable dose for at least 3 months and on stable doses of other PAH treatments for at least 30 days prior to screening will be enrolled. After a screening visit, eligible subjects will have a 4-week run-in period with their existing FLOLAN (epoprostenol sodium) treatment. At the end of the 4-week period, they will be admitted to the clinic for baseline assessments and for switching to study medication (the new thermo stable formulation of epoprostenol sodium). Subjects will remain in hospital for a minimum of 6 hours to ensure clinical and hemodynamic stability prior to discharge. Subjects may stay in hospital for up to 24-48 hours after switching to the new thermo stable formulation of epoprostenol sodium at the discretion of the investigator. Dose titration requirement will be assessed at the time of discharge. Haemodynamic parameters will be obtained in a subgroup of subjects enrolled in centres where the collection of haemodynamic data is considered part of the standard of care. Subjects will receive the study medication as a continuous intravenous infusion for a 4-week treatment period. Those who complete the 4-week treatment period will have the option of entering an extension phase of the study to continue receiving the new formulation.