Hypoadrenocorticism produces more severe hypotension during the peripartal period in pregnant ewes and women. We hypothesized that estradiol increases the severity of hypotension after withdrawal of corticosteroids, and that this results from combined effects of adrenalectomy and estradiol to increase endothelial nitric oxide synthase (eNOS). In Study I, blood pressure and eNOS mRNA and protein in aorta, uterine, renal and mesenteric arteries were measured in intact ewes or adrenalectomized ewes 18-20h after cessation of infusion of cortisol and aldosterone; half of each group were treated with estradiol. In Study II, adrenalectomized ewes were similarly studied 22-28h after withdrawal of corticosteroids. Estradiol treatment in both studies significantly increased eNOS mRNA and protein in uterine artery, whereas corticosteroid withdrawal decreased expression of eNOS mRNA and protein in uterine artery. In both studies adrenalectomy and steroid withdrawal decreased mean arterial pressure. In Study II, 4 of 6 adrenalectomized ewes not treated with estradiol showed dramatic phasic variations in blood pressure and heart rate with a period of ~20 sec, developing within 22-28 h after corticosteroid withdrawal. Although there was no effect of estradiol on blood pressure in Study I, in Study II ewes treated with estradiol did not develop this pattern. Estradiol also slowed both the decline in plasma sodium and the rise in plasma potassium after corticosteroid withdrawal. These results disprove the hypothesis that estradiol increases the severity of hypotension during hypoadrenocorticism. However the study reveals an important effect of corticosteroid withdrawal on blood pressure, consistent with corticosteroid modulation of baroreflex responsiveness.