Monoamine oxidases (MAOs) are mitochondrial enzymes with 2 isoforms that have emerged as important contributors to cardiovascular oxidative stress via the constant generation of hydrogen peroxide. The present study was purported to assess whether MAO-derived H2O2 contributes to the endothelial dysfunction in mammary arteries harvested from coronary heart disease patients with/without diabetes mellitus subjected to coronary artery bypass grafting. To this aim the effects of MAO inhibition on vascular contractility to phenylephrine and endothelial-dependent relaxation (EDR) in response to acetylcholine were studied in vascular segments. Clorgyline (irreversible MAO A inhibitor), selegiline (irreversible MAO B inhibitor), and moclobemide (reversible MAO A inhibitor) were applied in the organ bath (10Âľmol/L). MAO expression was assessed by immunohistochemistry. We found a constant impairment of EDR that has been significantly attenuated in the presence of the MAO A and B inhibitors in both groups of coronary heart disease patients. MAO B was the dominant isoform in all human diseased vessels. In conclusion, in vitro inhibition of MAO significantly improved endothelium-dependent relaxation in human mammary arteries, regardless the presence of diabetes. These data suggest that MAO inhibitors might be useful in restoring endothelial response in clinical conditions associated with increased oxidative stress, such as coronary artery disease and diabetes. ; The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author.