Hypothesis1 (H1): a) EDD will increase in response to BH4 in early peri, late peri and postmenopausal women but not premenopausal women. b) The increase in EDD in response to BH4 will be associated with a decrease in plasma markers of oxidative stress. c) The reduced EDD in peri- and postmenopausal women will be associated with lower endothelial cell markers of NO and higher oxidative Stress.