Recently, it could be shown that the renin-angiotensin-system (RAS) influences different signal transduction pathways within the red blood cells (RBC). This includes the Na+/H+ exchange activity, the Ca2+-ATP-ase mediated Ca2+efflux, the erythropoietin- dependent production of RBC, the RBC deformability, RBC aggregation and the interaction of RBC and platelets. Recent studies and experiments, done by our group, focus on the oxidative and nitrosative metabolism of NO within the blood. The interactions of the RAS and endothelial NO are well known and described in detail. Based on a wide experience in this research field of NO metabolism, we characterized recently an active endothelial-type NO-synthase in RBC on the biochemical, functional and molecular level. Erythrocyte-derived NO formation serves important regulatory functions essential for adequate passage of blood through the vasculature. Here we aimed to treat patients with diabetes mellitus and arterial hypertension with Telmisartan as an angiotensin receptor antagonist. Efficacy parameters studied in this study should be: i) RBC deformability, RBC aggregation, ii) RBC dependent production of nitric oxide as well as detection of eNOS activity in RBC and iii) indices of microcirculatory perfusion. This project could broaden the view of effects of Telmisartan in the treatment of microcirculatory disorders in patients with diabetes mellitus and arterial hypertension, who exhibit a reduced NO bioavailability and RBC function.