In young men and women, melatonin influences vascular reactivity and reduces blood pressure and norepinephrine levels. Herein, we investigated whether these effects are conserved in postmenopausal women without and with hormone replacement therapy (HRT). Oral melatonin (1 mg) or placebo was randomly and in double blind fashion administered to 18 untreated and 13 postmenopausal women who were treated continuously with transdermal estradiol (50 microg/day) plus cyclic medroxyprogesterone acetate (5 mg/day x 12 days every 28 days). Internal carotid artery pulsatility index (PI), an index of downstream resistance to blood flow, blood pressure and catecholamine levels were evaluated. In untreated postmenopausal women, melatonin was ineffective, while in HRT-treated women, studied during the only estrogenic phase, melatonin reduced, within 90 min, systolic (-8.1 +/- 9.9 mmHg; P = 0.054), diastolic (-5.0 +/- 7.0 mmHg; P = 0.049) and mean (- 6.0 +/- 6.6 mmHg; P = 0.037) blood pressure. Norepinephrine (-50.1 +/- 66.7 pg/mL; P = 0.019), but not epinephrine levels, were also significantly reduced. Similarly, resistance to blood flow in the internal carotid artery, as evaluated by the PI, decreased (-0.190 +/- 0.15; P = 0.0006) in a way that was linearly related to pre-existing PI values (r2 = 0.5; P = 0.0059). These data show that the circulatory response to melatonin is conserved in postmenopausal women on HRT but not in untreated postmenopausal women. Possible physiological and pharmacological implications of these data on the cardiovascular risk of postmenopausal women can be envisioned.