As women get older and go through menopause, levels of the female reproductive hormone estradiol decrease to low levels. Also with aging, the functioning of the arteries declines. Over time this vascular dysfunction can lead to health problems such as high blood pressure and heart disease. This study is being done to help determine what causes arteries to become unhealthy in postmenopausal women, who have low levels of the female reproductive hormone estradiol. In this study we will test whether low levels of tetrahydrobiopterin (BH4), a natural substance in the body that can cause the arteries to expand, explains why arteries become unhealthy in women with low levels of estradiol. To answer this question, we will study how vascular function changes with a medication that causes a short-term increase in BH4 levels when estradiol is lowered with a medication, compared to when estradiol is normal. We will also determine whether the administration of the antioxidant vitamin C, along with the medication to increase BH4 levels, will normalize vascular health in perimenopausal and postmenopausal women, and in women who have their estradiol levels lowered.