This study will determine the effects of hormone replacement therapy (estrogen alone or estrogen and progesterone) on the walls of arteries in postmenopausal women. Inflammation and stiffness of artery walls are two risk factors for atherosclerosis-deposits of fatty substances (plaques) that can block the vessel, causing a heart attack or stroke. Estrogen raises the levels of certain substances in the blood that cause vessel inflammation and lowers the levels of others. This study will measure the net effects of estrogen on artery wall inflammation and stiffness.