This pilot research will be conducted in 64 post-menopausal women participating in an existing study of vitamin D supplementation (32 will receive vitamin D3 2,500 IU daily, the others matching placebo) and will explore the effects of vitamin D on endothelial function and arterial reactivity. Post-menopausal women aged 55-65 years are chosen due to their highest risk for development of a subsequent new cardiovascular disease diagnosis. All study participants will have fasting laboratory and noninvasive vascular ultrasound studies performed at baseline and four months later. The primary outcome measure of this pilot study is change in markers of endothelial function and arterial stiffness with vitamin D3 therapy. If our hypotheses are correct, our long-term goals include investigation of the effect of vitamin D repletion on subclinical atherosclerosis and subsequent cardiovascular events.