This is a multi-center, open-label pharmacokinetic (PK) study examining the relationship between the steady-state plasma concentration and dose of treprostinil delivered intravenously or subcutaneously in children with pulmonary arterial hypertension (PAH). Subjects will be divided into 5 cohorts by age. A blood sample will be obtained from each subject at steady state. Additional blood samples will be obtained from a small subset of subjects with a 15% increase or with at least a 15ng/kg/min increase in dose from steady state. Samples will be sent to a pharmacokinetic laboratory for analysis. Linear regression analysis will be used to determine the relationship between the steady state plasma concentration and drug dose. A power model will be used to assess dose proportionality.