Postmenopausal women in good health may participate in this study. Volunteers will be screened for eligibility with a complete medical history, heart examination, and blood tests. Participants will be randomly assigned to receive either: 1) hormone therapy (estradiol 2 mg daily alone for women who have had a hysterectomy or estradiol plus micronized progesterone 200 mg daily for women with an intact uterus); or 2) placebo (look-alike pills that contain no active drug). Women in both groups will take pills for 3 months, then no pills for 1 month, and then will crossover to the alternate therapy for 3 months (i.e., those in the original placebo group will take hormones, and those in the hormone group will take placebo). At the end of each 3-month treatment period, participants will undergo the following procedures to assess blood vessel inflammation and stiffness: