Studies have reported over-activation of the sympathetic nerve system (SNS) in patients with idiopathic Pulmonary Arterial Hypertension (iPAH). Since the Renin-Angiotensin-Aldosteron-System (RAAS) is closely related to SNS and the lungs are the major site for angiotensin 2 formation, we hypothesized that RAAS-activity is increased in iPAH. Pulmonary endothelial cell (P-EC) cultures were generated from lung specimens of iPAH-patients (n=5) and controls (n=5) to assess angiotensin converting enzyme (ACE) activity (Fig. 1A). We determined angiotensin 2 production upon incubation with angiotensin 1 alone and in combination with ACE-inhibitor enalapril. Subsequently, protein levels of the angiotensin 2 receptor type 1 (AT1R) and tyrosine kinase SRC-activity (downstream target of AT1R) were evaluated in pulmonary arteries (PA) homogenates. P-EC of iPAH-patients produced significantly more angiotensin 2 upon angiotensin 1 incubation, compared to control. Interestingly, enalapril normalized angiotensin 2 production (Fig. 1B). In addition, pulmonary arteries of iPAH-patients exhibited increased AT1R expression and accentuated SRC-activity (Fig. 1C,D) Conclusion: This study demonstrates increased RAAS-activity in lungs of iPAH-patients, illustrated by elevated ACE-activity and AT1R signalling. Future studies will focus on the effects of chronic inhibition of RAAS on pulmonary vascular remodelling in iPAH.