Estrogens have been reported to influence endothelial functions. This article's aim is to examine whether endothelium-dependent vasodilation of brachial arteries is changed in parallel with the physiological variation of the ovarian hormones during the menstrual cycle in young healthy female subjects. With high-resolution ultrasonography, we measured the diameter and blood flow of brachial arteries at rest, during reactive hyperemia, and after sublingual nitroglycerin administration in 15 young healthy female subjects (mean 29.6 +/- 2.6 years). All female subjects were studied in each of three different phases of one menstrual cycle (M, menstrual phase; F, follicular phase; L, luteal phase). The increase in arterial diameter during reactive hyperemia was lowest in the menstrual phase and highest in the follicular phase (4.9 +/- 0.8% in M; 14.1 +/- 0.9% in F; 9.2 +/- 1.1% in L; p < 0.005 between any two means among the groups). No significant difference was apparent in the percentage of increase in blood flow during reactive hyperemia among the three phases. The percentage of increase in arterial diameter after nitroglycerin administration also was not significantly different among the three phases of the cycle. The serum estradiol level was lowest in the menstrual phase and was highest in the follicular phase (17.5 +/- 1.6 pg/ml in M, p < 0.0005, M versus F and L; 107.8 +/- 18.0 pg/ml in F, p < 0.05, F versus L; 74.4 +/- 7.3 pg/ml in L). The serum level of nitrite-nitrate, measured by the Griess reaction, was highest in the follicular phase and was lowest in the menstrual phase of the three different phases (40.4 +/- 7.9 mumol/l in M, p < 0.01, M versus F, and p < 0.05, M versus L; 106.7 +/- 17.6 mumol/l in F, p < 0.05, F versus L; 62.2 +/- 6.2 mumol/l in L). In conclusion, a significant variation of the endothelium-dependent vasodilation parallels the levels of serum estradiol and nitrite-nitrate during the menstrual cycle in female subjects.