Osteoporosis is a significant health problem in the US. accounting for approximately 1.2 M fractures each year with annual costs of $6.1 B. Traditional therapies are expensive require medical supervision, and sometimes have undesirable side effects. Recent NIH conferences stressed th need to develop safe, effective, low-cost strategies which might be applicable to populations at large for maximizing peak bone mass, minimizin bone loss, and preventing osteoporotic fractures. Direct evidence for an exercise effect on bone mineral content is lacking. There is a need for prospective studies in humans to determine the effectiveness of exercise training to increase peak bone mass and minimize bone loss. Previous studie are limited by one or more methodological problems possibly rendering their conclusions invalid, e.g., 1) failure to randomize exercise and control groups, 2) poor control of extraneous variables, 3) small sample sizes, 4) failure to adjust for subject attrition, 5) imprecise measurement technique , 6) poorly designed exercise programs, 7) limited age groups of subjects and bone measurement sites. In the proposed study, we intend to quantify change in both axial and appendicular bone mass resulting from two years of progressive resistance exercise and to follow the maintenance of the change for two years after training has ceased. The subjects will be 520 white females selected from four age groups (120-160 per age group): 1) 12 to 15 years. 2) 17 to 19 years, 3) 30 to 40 years, and 4) 45 to 55 years. Subject assignment to experimental (N-60) and control N-60) groups will be randomiz d and measurements of bone mineral content will be made at several sites usin single and dual photon absorptiometric techniques. The sample size of 60-80 subjects per group has been estimated to be sufficient to detect the expect d changes. Further, we intend to correlate the skeletal changes with indices of bone formation and resorption (serum bone Gla protein, alkaline phosphatase, urinary hydroxyproline), and other physiological factors, e.g. calciotropic hormones, sex steroids, and diet. Potential confounding effect of variable Ca intakes will be minimized by having all subjects take a 500 mg/day Ca supplement. The significance of the proposed study lies in attaining a better understanding of the effectiveness of exercise for increasing peak bone mineral mass and minimizing bone loss in different age groups, and the physiological factors indicating or mediating these effects This information is important to develop an effective exercise prescription for both the prevention and treatment of osteoporosis.