This study investigates whether in postmenopausal women ages 45-75 years, there is an age-related effect of transdermal estrogen and oral progestin therapy on hormonal mediators of bone mineral metabolism, specifically: 1) calciotropic hormones, including parathyroid hormone, calcitonin, and 1,25 dihydroxyvitamin D3, 2) basal and modulated growth hormone and basal insulin-like growth factor-1.