BACKGROUND: Estrogen replacement therapy (ERT) in postmenopausal women decreases cardiac mortality and improves endothelial function. The endothelium regulates vascular tone and growth by releasing such factors as nitric oxide and endothelin-1. OBJECTIVE: To determine whether ERT alters the balance between the total oxidized products of nitric oxide and endothelin-1. DESIGN: Single-arm, before-after clinical trial. SETTING: Outpatient clinical research center of an academic medical center. PATIENTS: 15 postmenopausal women. INTERVENTION: Treatment with 17 beta-estradiol for 6 months and a 10-day course of methoxyprogesterone every 3 months. MEASUREMENTS: Plasma nitric oxide and endothelin-1 levels were measured at baseline and after 6 months of ERT. RESULTS: The mean baseline nitric oxide level was 27.5 nmol/mL (range, 20.3 to 34.8 nmol/mL) and increased by a mean of 7.2 nmol/mL (range, 0.2 to 14.1 nmol/mL) (P = 0.04). The mean baseline plasma endothelin-1 level was 16.4 pg/mL (range, 12.0 to 20.8 pg/mL) and decreased by a mean of 3.9 pg/mL (range, 0.4 to 7.8 pg/mL) (P = 0.04). The mean baseline ratio of nitric oxide to endothelin-1 was 2.0 (range, 1.3 to 2.8) and increased by 1.2 (range, 0.1 to 2.2) (P = 0.03). CONCLUSION: ERT results in an increased ratio of nitric oxide to endothelin-1. This may be one mechanism by which ERT provides cardiovascular benefit.