. The primary prevention of breast cancer is an important public health goal in this country. Exercise has been associated with a decreased risk for breast cancer. One mechanism may be via a hormonal pathway. Exercise decreases body fat, especially abdominal fat, which may be the mechanism through which exercise could lower concentrations of endogenous estrogens in premenopausal women. This application is a randomized interventional study that will test the effect of a one-year, moderate intensity exercise program on androgenous sex hormone profile in postmenopausal woman. Women (N=168) who are age 55-75 years will be randomized to either a one-year, moderate intensity exercise program or a controlled program of no change in exercise habits. Dietary intake will be assessed at baseline and follow-up through four-day food records. Serum hormones will be assayed at baseline and at end-of-study. These hormones include: total estrone, total estradiol, free estradiol, percent bioavailable estradiol, estrone sulfate, sex hormone binding globulin, albumin, testosterone, free testosterone, androstenedione, dehydroepiandrosterone, dehydroepiandrosterone sulfate, insulin, glucose and triglycerides. The ratio of urinary 2-hydroxyestrone (2-OHE1) to 16a-hydroxyestrone (16-alpha-E1), an estrogen metabolite ratio which may be associated with breast cancer, will also be assayed at baseline and at end-of-study. The effect of exercise on weight, body mass index, total fat mass, and body fat distribution will also be assessed. This study plans to give insight into possible mechanisms through which exercise may be associated with decreased risk of breast cancer.