Abstract-Estrogen (E2) acts in the brain to decrease blood pressure (BP) responses to psychological stress. A likely site for the effects of E2 is the hypothalamic paraventricular nucleus (PVN), an important regulator of autonomic functions. We studied the effects of E2 in the PVN on BP and heart rate (HR) responses to L-glutamate injections into the PVN of male urethane-anesthetized rats. Microinjections of L-glutamate (50 nmol) into the PVN increased BP by 14Ϯ2.5 mm Hg and HR by 30Ϯ5.6 bpm. Microinjections of E2 (0.1, 1, and 10 pmol) into the PVN 30 minutes before L-glutamate dose-dependently attenuated the pressor response by 25%, 34%, and 59%, respectively, but did not affect HR. We determined that E2 receptor (ER)  mediates the effect of E2, because activation of ER with diarylpropionitrile (50 pmol) attenuated the response by 57%, whereas activation of ER␣ with propyl-pyrazole-triol (20 pmol) had no effect. Furthermore, inhibition of ER with R,R-tetrahydrochrysene (50 pmol) blocked the effect of E2, but inhibition of ER␣ with methyl-piperidino-pyrazole (1 nmol) did not. Finally, we found that the effect of E2 is mediated by NO, because the NO synthase (NOS) inhibitor, N G -nitro-L-arginine methyl ester (2 nmol), the neuronal NOS inhibitor, 7-nitroindazole sodium salt (0.1 pmol), and the endothelial NOS inhibitor, N5-(1-iminoethyl)-L-ornithine (200 pmol) blocked the effect of E2. The effect was partially blocked with the ␥-aminobutyric acid A receptor inhibitor bicuculline. Our results demonstrate that E2 in the PVN attenuates the L-glutamate-induced pressor response and that this effect is mediated by ER, NO produced by neuronal NO synthase and eNOS, and partly by ␥-aminobutyric acid. Key Words: 17-estradiol Ⅲ blood pressure Ⅲ estrogen receptor Ⅲ nitric oxide synthase Ⅲ autonomic nervous system T he increased incidence of cardiovascular disease in postmenopausal women has been attributed to the loss of estrogen (E2), 1,2 and it was observed that hormone replacement therapy confers cardiovascular benefits in these women. [1][2][3] In 1993, the Women's Health Initiative trial began recruiting Ͼ16 000 postmenopausal women to assess the benefits and risks of hormone replacement therapy on the incidence of heart disease, cancers, and fractures. 4 Surprisingly, the trial was terminated in 2002 because of increased risks of breast cancer and stroke. 4 Although the Women's Health Initiative trial has been criticized for its design and conclusions, 2,5 the trial illustrates the need to better understand the effects of E2 on the cardiovascular system and on general physiology to develop specific therapies that will contribute to cardiovascular health in women and men.E2 provides cardiovascular benefits in response to stress. E2 decreases sympathetic activity 6 and blood pressure (BP) responses to mental stress in postmenopausal women. 7,8 We and others have shown that E2 attenuates BP and HR responses to restraint stress 9,10 and cage switch 10 in ovariectomized (OVX) rats. E2 in the brain also modulates cardio...