Background: Monoamine oxidases (MAOs) are mitochondrial dehydrogenases with two isoforms (A and B) which catalyze the electron transfer from biogenic amines to molecular oxygen, constantly generating hydrogen peroxide (H2O2) as by-product. The present study was purported to asess whether MAOs-derived H2O2 contributes to vascular dysfunction and MAO inhibition improves endothelial-dependent vasodilation in diabetic rats. To this aim, the effects of MAO-A inhibitor (clorgyline, selegyline and moclobemide) on endothelium-dependent relaxation (EDR) in response to acetylcholine have been studied in phenylephrine-preconstricted aortic segments isolated from rats with streptozotocin induced diabetes (STZ). MAO expression was assessed by quantitative RT-PCR and immunohistology. Measurements of H2O2 production were performed using the ferrous iron xylenol orange oxidation method. Results: In diabetic vessels EDR was signifi cantly decreased whereas H2O2 generation was found to be increased. Incubation with MAO inhibitors attenuated H2O2 production by 50% (nM/mg tissue/h: Control: 0.23±0.03, STZ= 0.85±0.05, Control+Selegyline: 0.22±0.02, STZ+Selegyline: 0.45± 0.04, n=10; p<0.05 STZ vs CTL; p<0.05 STZ with vs without Selegyline) in vessels from diseased animals and partly restored normal EDR. Conclusions: MAO-A and B are both expressed in rat aortas being increased in diabetic vessels and contribute via H2O2 generation to the impairment of vascular function. MAO is a novel mediator of endothelial dysfunction in experimental diabetes. Keywords: monoamine oxidase, endothelial dysfunction, experimental diabetes.