Inclusion Criteria:

* Diagnosis of idiopathic PAH (formally known as PPH), or PAH associated with collagen vascular disease, anorexigen use, or HIV infection;
* Historical cardiac catheterization with the following hemodynamic criteria:

Mean pulmonary artery pressure greater than or equal to 25 mmHg; Pulmonary vascular resistance greater than 3 mmHg/L/min; Pulmonary capillary wedge pressure or left ventricular end diastolic pressure less than 15 mmHg;

* 6-minute walk distance of at least 150 meters, but no more than 450 meters;
* Total lung capacity greater than or equal to 70% and FEV1 greater than or equal to 65% of predicted normal;

Exclusion Criteria:

* Portopulmonary hypertension;
* Subjects with PAH due to or associated with coronary artery disease, left heart disease, interstitial lung disease, chronic obstructive pulmonary disease, veno-occlusive disease, chronic thromboembolic disease, or sleep apnea;
* Bosentan (Tracleer®), sildenafil (Viagra®), or chronic prostanoid therapy within 4 weeks of screening;
* Serum ALT or AST lab value that is greater than 1.5 times the upper limit of normal;
* Contraindication to treatment with an endothelin receptor antagonist;
* Subject with cardiovascular, hepatic, renal, hematologic, gastrointestinal, immunologic, endocrine, metabolic, or central nervous system disease that may adversely affect the safety of the subject;
* Participation in a clinical study involving another investigational drug within 4 weeks of screening.