During the menstrual cycle, changes in endothelium-dependent vasodilatation have been demonstrated in conduit vessels in vivo, but responses in resistance vessels have not been studied. The aim of this study was to examine endothelium-dependent vasodilatation, the effects of local nitric oxide synthesis, and alpha-adrenergic constriction in resistance vessels during the menstrual cycle in 15 healthy female volunteers (mean age, 28.07 ± 2.1 yr). Forearm blood flow in response to intrabrachial infusion of bradykinin (10, 30, and 100 pmol/min; endothelium-dependent vasodilator), glyceryl trinitrate (4, 8, and 16 nmol/min; endothelium-independent vasodilator), noradrenaline (60, 120, and 240 pmol/min; alpha-adrenergic receptor agonist), and NG-monomethyl-L-arginine (1, 2, and 4 micromol/min; nitric oxide synthase inhibitor) was assessed by venous occlusion plethysmography. All subjects were studied in early menstrual phase (days 1--4) and midcycle (days 10-13). Vasodilator response to bradykinin, expressed as the within-subject mean difference in the area under the dose-response curve between phases, was significantly increased at midcycle compared with that in the early menstrual phase (486.5 ± 165.0; P = 0.01), whereas there was no significant difference in response to glyceryl trinitrate (185.8 ± 239.0; P = 0.45). The vasoconstrictor response to noradrenaline was significantly greater at midcycle (97.1 ± 39.4; P = 0.027), but the response to N(G)-monomethyl-L-arginine was not significantly different (17.5 ± 35.2; P = 0.63). Serum estradiol was approximately 3-fold higher at midcycle, with a mean difference of 252.3 ± 56.0 pmol/L (P = 0.0005). Progesterone concentrations were not significantly different (-0.11 ± 0.1 nmol/L; P = 0.28). Differences in endogenous estrogen levels between menstrual phases may underlie changes in bradykinin and noradrenaline responses. If exogenous estrogens have similar effects, the balance of these two opposing actions may determine whether estrogen replacement in postmenopausal women has beneficial or harmful effects on the vasculature.