Recruitment of circulating monocytes into the vasculature and release of reactive oxygen species (ROS) promote atherogenesis. Rac1-GTPase is an essential component of the superoxide-producing NADPH-oxidase complex. Estrogens inhibit production of vascular reactive oxygen species. Angiotensin II as well as overexpression of the constitutively active Mutant RacL61 increased ROS production in monocytes. AngII-mediated ROS release was completely inhibited by overexpression of the dominant negative mutant RacN17 or treatment with 17 beta-estradiol. 17 beta-Estradiol reduced Rac1-expression concentration- and time-dependently and decreased basal, as well as AngII-induced Rac1 activity. The effects of 17 beta-estradiol were receptor-mediated. In vivo, down-regulation of Rac1 by 17 beta-estradiol was observed in human mononuclear cells of women with elevated 17 beta-estradiol levels after controlled ovarian hyperstimulation. In summary, the data show that clown-regulation of Rac1-GTPase contributes to the inhibition of angiotensin II-mediated superoxide release by 17 beta-estradiol in monocytes. (C) 2009 Elsevier Inc. All rights reserved.