Endothelial dysfunction, characterized by enhanced levels of ROS and reduced NO bioavailability, as well as increased thrombogenicity has been associated with atherosclerosis. Progression of atherosclerosis has been positively correlated with angiogenesis.Thrombin has been shown to promote angiogenesis and to enhance ROS and NO levels. We therefore investigated whether NO and the GTPase Rac, known to activate NADPH oxidases, are involved in the angiogenic response to thrombin in microvascular endothelial cells (EC). Stimulation with thrombin enhanced ROS and NO production, increased Rac activity, protein expression and activation of eNOS. Transfection of the active RacG12V mutant elevated ROS production, whereas the dominant-negative RacT17N had opposite effects. In contrast, inhibition of eNOS by L-NNA enhanced ROS production and Rac activity whereas antioxidants prevented them. Thrombin, RacG12V and L-NNA stimulated angiogenesis whereas antioxidants and RacT17N diminished it. Finally, L-NNA-stimulated ROS production, Rac activity and angiogenesis were decreased by antioxidants.These data show that the angiogenesis is promoted by a redox-sensitive pathway in thrombin-stimulated EC which is counterbalanced by NO. Thus, activated Rac may play an important role in linking oxidative stress with a procoagulant state and angiogenesis in atherosclerosis.