Endothelial cells are key modulators of diverse physiological processes, and their impaired function is a cause of numerous cardiovascular diseases. Under physiologic condition, the reactive oxygen and nitrogen mediators in endothelia lead to the signal propagation of the initial stimulus, by forming molecules with a longer half-life like hydrogen peroxide. Hydrogen peroxide is the focus of growing attention in endothelial biology, and consequently the enzymes involved in its generation and clearance are viewed as novel mediators of great importance. In particular, among peroxidases, myeloperoxidase is recognized as a key enzyme, capable of impairing intracellular NO reservoirs as well as producing oxidized amino acids such as 3-chlorotyrosine or 3-nitrotyrosine. This process switches the functional pathways from normal signalling to a condition characterized by oxidative and/or nitrosative stress. Understanding the molecular mechanisms involved in these stress responses in endothelium will lead to better therapeutic strategies for oxidative stress-driven cardiovascular diseases.