--- - |2+ Abstract Aim We tested the hypothesis that estradiol (E2) reduces aortic oxidative stress and endothelial dysfunction in ovariectomized (OVX) female rats exposed to room air (RA) or chronic intermittent hypoxia (CIH). Methods We used intact or OVX female rats treated with vehicle or E2 (0.5 mg/kg/d) and exposed to RA or CIH (21%‐10% O2, 10 cycles/h, 8 h/d) for 7 or 35 days, and measured the arterial pressure, heart rate and plasma endothelin‐1 levels. We also measured in thoracic aortic samples, the activities of the pro‐oxidant enzymes NADPH (NOX) and xanthine oxidase (XO), the antioxidant enzymes superoxide dismutase, catalase, glutathione peroxidase and the advanced oxidation protein products (AOPP—oxidative stress marker). Finally, we used aortic rings to assess the contractile response to phenylephrine and the vasodilatory response to acetylcholine. Results After 7 or 35 days of CIH, E2 supplementation reduced arterial pressure. E2 reduced plasma endothelin‐1 levels after 7 days of CIH, but not after 35 days. Ovariectomy, but not CIH for 7 days, increased aortic oxidative stress and E2 treatment prevented this effect. Remarkably, in animals exposed to RA, this was achieved by a reduction in NOX and XO activities, but in animals exposed to CIH this was achieved by increased catalase activity. In OVX female rats exposed to CIH for 7 days, E2 supplementation improved the NO‐mediated vasodilation. After 35 days of CIH, enzymatic activities, AOPP and aortic reactivity were similar in all groups. Conclusion E2‐based therapy could help prevent the vascular consequences of CIH in apneic women. - 'Acta Physiologica, EarlyView. '