Abstract. The renin-angiotensin-aldosterone system (RAAS) has always earned a key position in the regulation of blood pressure either under physiological conditions or in the pathogenesis and maintenance of arterial hypertension. With the approval of aliskiren – a renin antagonist that is meant to directly bind to the protease renin – the possibilities of interfering with the RAA system have been enhanced. However, a comprehensive in vitro characterization of the effects of aliskiren is limited. This seems particularly important since the results regarding systemic effects and side effects in a clinical setting are not that explicit. To this end, we aim to characterize the effects of aliskiren on vascular function in an in vitro model of vascular tissues. To this purpose, we investigated the effects of aliskiren in ascending concentrations (1 – 50 µM) on a range of vascular smooth muscle preparations, i.e., iliac artery, aorta, and portal vein, in a conventional organ bath. While we found vasodilatatory effects of aliskiren for the preparations of the iliac artery and aorta, the portal vein showed a concentration-dependent biphasic response with an inhibitory component at lower concentrations and excitatory effects with an increase of mechanical activity and frequency of spontaneous activity in higher concentrations. Our findings extend the literature regarding aliskiren and suggest an effect that is independent of renal inhibition of renin.