Discordance between the known biology of estrogen and the results of clinical trials of postmenopausal hormone therapy underscore the need to better characterize, in controlled mechanistic studies, estrogens multiple actions in human health and disease. Exercise-associated amenorrhea (EAA) is a common menstrual abnormality associated with insufficent caloric intake, resulting in decreased estrogen concentrations similar to that seen in postmenopausal women. Since the premenopausal years are associated with cardioprotection, and determine the postmenopausal CV disease burden, investigation of CV function in EAA women are of particular interest and relevance. A prospective, investigator-blind, cross-over controlled mechanistic study will be conducted. Healthy, young (18-35 yrs) women will be allocated to one of the following groups: 1) sedentary, regularly menstruating and ovulatory; 2) exercising regularly menstruating and ovulatory; and 3) EAA. Measures of vascular and autonomic function, and serum indices of vascular inflammation and oxidative stress will be undertaken. Women with EAA will be tested before and after 4 weeks of transdermal estrogen. The purpose of this study is: 1) to assess the effects of estrogen on vascular function and oxidative stress, as well as neurohumoral mechanisms of vascular function and blood pressure regulation; and 2) to compare these responses in women with EAA in an estrogen deplete and replete environment.