Changes in vascular nitric oxide (NO) activity may contribute to cardiovascular risk. We determined the effect of the menopause, gender, and estrogen replacement therapy on arterial vascular NO activity. Vascular NO activity and sensitivity were determined in 15 healthy premenopausal women (mean age, 48 yr), 12 postmenopausal women (51 yr), and 14 men (51 yr). The effects of 14 days of estrogen replacement therapy (625 microg conjugated estrogens) were studied in 20 healthy postmenopausal women (60 yr). Forearm blood flow responses to brachial arterial infusions of L-NG-monomethyl-arginine (L-NMMA), norepinephrine, glyceryl trinitrate (GTN), and serotonin were determined using venous occlusion plethysmography. Constrictor responses to L-NMMA were reduced in postmenopausal women (82 +/- 14, summary response, mean +/- SEM) and men (89 + 6) compared to premenopausal women (118 + 10; P < 0.05). Constrictor responses to norepinephrine were increased in males (125 +/- 13) compared to premenopausal (81 +/- 8) and postmenopausal (88 +/- 16) women (P < 0.05). No differences were observed in GTN or serotonin responsiveness. Constrictor responses to L-NMMA increased after estrogen replacement (132 +/- 7 vs. 89 +/- 14; P < 0.05), with no change in norepinephrine, GTN, or serotonin responses. The menopause and male gender were associated with reduced arterial NO activity. Two weeks of estrogen replacement therapy restored vascular NO activity to premenopausal levels. Changes in vascular NO activity may contribute to changes in cardiovascular risk associated with male gender, postmenopausal status, and estrogen replacement therapy. Increased alpha-adrenoceptor responsiveness may contribute to increased cardiovascular risk in males.