To evaluate the effect of estrogen therapy (ET) on endothelial nitric oxide (N O) production and in flow-mediated vasodilation (FMV). Randomized, crossover, d ouble-blind, placebo-con-trolled study. Healthy postmenopausal women in an ac ademic research environment. Forty postmenopausal women between 45 and 72 years of age. Women received ET or placebo during two periods of 12 weeks that were se parated by 2 weeks of washout. Flow-mediated vasodilation, nitrite and nitrate, lipid profile, creatinine, and glucose were measured at weeks 12 and 24. Studen t’s t or Wilcoxon tests were used for comparative analyses, and κ.test and lim it analysis determined variability. After placebo treatment, nitrate and nitrite mean concentration was 8.28 ±1.17 mmol/L; it increased to 62.6 ±12.82 mmol/L after ET. Percentage FMV was 18.8 ±2.58 after the placebo period and did not ch ange after ET (20.1 ±1.92) in the whole sample, but in the subgroup (n = 15) of younger women (4550 years of age), percentage FMV increased from 13.6 ±3.6 aft er the placebo period to 22.2 ±3.5 after ET. An increase in plasma concentratio ns of nitrite and nitrate after ET was observed in all the women studied, but th e improvement in FMV was observed only in the younger ones. These age-related d ifferences in FMV in response to ET must be further investigated. To evaluate the effect of estrogen therapy (ET) on endothelial nitric oxide (NO) production and in flow-mediated vasodilation (FMV). Randomized, crossover, double-blind, placebo-con-trolled study. Healthy postmenopausal women in an ac ademic research environment. Forty postmenopausal women between 45 and 72 years of age. Women received ET or placebo during two periods of 12 weeks that were sected by 2 weeks of washout. Flow-mediated vasodilation, nitrite and nitrate, lipid profile, creatinine, and glucose were measured at weeks 12 and 24. Studen t’s t or Wilcoxon tests were used for comparative analyzes, and κ.test and lim it analysis determined variability. After placebo treatment, nitrate and nitrite mean concentration was 8.28 ± 1.17 mmol / L; it increased to 62.6 ± 12.82 mmol / L after ET. Percentage FMV was 18.8 ± 2.58 after the placebo period and did not change after ET (20.1 ± 1.92) in the whole sample, but in the subgroup (n = 15) of younger women (4550 years of age), per centage FMV increased from 13.6 ± 3.6 aft er the placebo period to 22.2 ± 3.5 after ET. An increase in plasma concentration of nitrite and nitrate after ET was observed in all the women studied, but th e improvement in FMV was observed only in the These age-related d ifferences in FMV in response to ET must be further investigated.