SA1: To determine the possible mechanistic involvement of BH4 deficiency in the decline in endothelial function during the peri-and early postmenopausal period and its potential relation to oxidative stress. This will be assessed by comparing brachial artery EDD and plasma and endothelial cell markers of oxidative stress and NO (see methods for details) in healthy pre-, peri-and postmenopausal women: 1) under basal conditions; and 2) in response to acute oral BH4 (vs PL) supplementation.