Background and Purpose-Female, compared with male, animals are protected from cerebral ischemic injury.Physiological concentrations of 17-estradiol (E2) reduce damage in experimental stroke. E2 augments angiogenesis in reproductive organs and noncerebral vascular beds. We hypothesized that E2 protects brain in stroke through modulation of angiogenesis. We quantified molecular markers of angiogenesis and capillary density before and after unilateral middle cerebral artery occlusion (MCAO). Methods-Female animals were ovariectomized, treated with 25 g E2 or placebo implants, and subjected to 2-hour MCAO and 22 hours of reperfusion. Brain angiopoietin-1 (Ang-1), Ang-2, Tie-1, Tie-2, vascular endothelial growth factor (VEGF), VEGF R1, and VEGF R2 mRNA levels were determined by RNAse protection assays, and CD31-positive vessels were counted. Results-E2, but not ischemia, upregulated cerebral Ang-1 mRNA by 49%. Capillary density was higher in the brains of E2-treated animals. In estrogen receptor-␣ knockout (ERKO) mice, E2-mediated induction of Ang-1 mRNA was absent relative to wild-type littermates. Conclusions-These results suggest that E2 increases Ang-1 and enhances capillary density in brain under basal conditions, priming the MCA territory for survival after experimental focal ischemia. Key Words: angiogenesis Ⅲ cerebrovascular accident Ⅲ endothelial growth factors W omen are protected from stroke relative to men; this advantage is lost with menopause. In 2002, the combined estrogen-progestin arm of the Women's Health Initiative was stopped because of lack of efficacy in cardiovascular disease prevention, including stroke. 1 The estrogen-alone arm of the trial was discontinued because of increased stroke risk in hormone users. 2 Therefore, data from animals and cultured cells are under intense scrutiny to elucidate biological pathways of the effects of 17-estradiol (E2) in brain and cerebral vasculature. The cellular and molecular pathophysiology of cerebral ischemia is strongly influenced by gender and female sex steroids. Although the preventative role of E2 is unclear, it does reduce stroke sensitivity (ie, attenuates neuronal damage once ischemia occurs). [3][4][5][6] The mechanism of protection is likely multifactorial. 4 E2 signals through estrogen receptor-␣ (ER␣) and ER to alter gene expression. 4 Rodent brains express both receptors, but ER␣ functions in E2-mediated protection in stroke. 7 A candidate mechanism of cerebral protection is the ability of E2 to modulate angiogenesis. 8 -11 E2-mediated angiogenesis occurs in normal tissues (eg, female reproductive tract 8,9,12 , retina, 10 and other nonreproductive tissues) 11 as well as pathologic conditions (eg, experimental limb ischemia, 13 tumors, 8 and aging brain). 14 Enhancement of angiogenesis is an attractive property. Improvement of collateral blood flow through angiogenesis or other endothelial-based adaptive mechanisms may occur after transient ischemic attacks, before stroke, in patients with intracranial stenosis. 15 Such patients may be pr...