Patients with Pulmonary Arterial Hypertension (PAH) must be stable on therapy for at least 3 months prior to enrollment in the trial. We will include patients with idiopathic PAH and Familial PAH as well as PAH associated with collagen vascular disease or drug or toxin exposure. With the exception of PAH, subjects must be free of major medical illnesses, including diabetes mellitus (must have fasting plasma glucose < 126 mg/dL and taking no anti-hyperglycemic agent), malignancy or significant hepatic or renal disease. Subjects may be hypertensive and on anti-hypertensive medications as long as blood pressure is < 150/100 mm Hg. Subjects may also be dyslipidemic and/or taking drugs to improve abnormalities of lipid metabolism, but they will be excluded if they are taking medications known to alter insulin sensitivity, including glucocorticoids, niacin, anti-retrovirals, thiazolidinediones, or metformin. Use of oral contraceptives or estrogen and/or progesterone replacement therapy is permitted. Weight must be stable and the subjects agree not to change their eating habits or exercise regimen during the study period. There will be no restrictions with regard to race or socioeconomic status, and the racial/ethnic composition of the study population will be reflective of the communities surrounding the Stanford University Medical Center.