Amyloid beta-peptide (Abeta) cytotoxicity, the hallmark of Alzheimer's disease, implicates oxidative stress in both neurons and vascular cells, particularly endothelial cells. Consequently, antioxidants have shown neuroprotective activities against Abeta-induced cytotoxicity. Among the different antioxidants used in both in vitro and in vivo studies, 17beta-oestradiol (E2) has garnered the most attention. Oestrogen attenuated Abeta(E22Q)-induced toxicity in neurons but failed to protect endothelial cells. Here we show that E2-mediated activation of endothelial nitric oxide synthase (eNOS) increases the production of nitric oxide (NO), which, under Abeta(E22Q)-induced oxidative damage, results in the formation of peroxynitrite and increased nitration of tyrosine residues. Inhibition of eNOS prevents nitrotyrosination and permits E2-mediated protection against Abeta(E22Q) on endothelial cells. The main nitrotyrosinated proteins in the presence of E2 and Abeta(E22Q) were identified by MALDI-TOF mass spectrometry. These proteins are key players in the regulation of energy production, cytoskeletal integrity, protein metabolism and protection against oxidative stress. Our data highlight the potential damaging consequences of E2 in vascular disorders dealing with oxidative stress conditions, such as cerebral amyloid angiopathy, stroke and ischaemia-reperfusion conditions.