Tetrahydrobiopterin (BH4) is a critical co-factor in the production of NO by endothelial nitric oxide synthase (eNOS), and in the face of deficient levels, uncoupling of eNOS results in the production of ROS instead of NO. Whether the increased ROS and impaired EDD that occurs during the peri- and early postmenopausal periods is associated with reduced vascular BH4 is at present unknown. However, our finding that AA could not improve the reduced EDD found in early perimenopausal women or restore EDD in peri-and postmenopausal women to premenopausal levels, indicates that other sources of ROS or causes of low NO are involved in the impaired EDD across the stages of the menopause transition and aging in women. In vitro studies demonstrate that the co-administration of BH4 and AA completely prevents the eNOS uncoupling and decreased NO production in endothelial cells treated with a peroxynitrite (ONOO-) donor, compared to administering either one alone. The global aim of this renewal is to follow-up on our previous findings and to determine the role of BH4 as a mediator of endothelial dysfunction [across the stages of the menopausal transition and aging in women. This global aim will be addressed by 3 specific aims (SA). Using a cross-sectional design, SA1 will measure EDD in healthy pre-, peri- and postmenopausal women: 1) under basal conditions; and 2) following an acute increase in BH4. SA2 will expand on the cross-sectional comparisons via short-term (10 days) ovarian suppression (GnRhant) in pre-and early perimenopausal women to isolate the age-independent effects of E2. SA3 will examine whether co-administration of BH4 and AA fully restore youthful EDD in peri-and postmenopausal women.