Exercise has often been used in the aged (65 plus) for treatment of osteoporosis. While its effectiveness as a modality for bone accretion has been demonstrated in our recently completed three year study, minimal research has been done on younger women, ages 40 to 65 years. This period of life represents the most critical bone mineral loss rate for the aging female. Therefore, we propose the use of a physical activity regimen for prevention and maintenance of bone loss in middle aged women over a five year period for 90 females. Eighty women of the same economic and social status and age (40 to 65 years) who are currently participating in a four year study in our Biogerontology Laboratory, will be used as a control group. Bone mineral changes will be monitored with I-125 absorptiometry equipment developed at the University of Wisconsin. Each subject will be measured bilaterally at the mid-shaft of the radius, ulna, and humerus. The measurements will be done in ten sessions over a span of four years at three-month intervals the first year and twice yearly thereafter. Duplicate measurements will be made at the initial and final visits. In addition, hand-wrist x-rays will be taken of the subjects at the beginning and end of the study for possible cortical thickness changes. The protocol will include a work capacity test at the beginning, second year, and end of the study for the control and exercise groups. Correlations will be made on the exercise group between bone mineral changes, VO2, and muscular strength. Subjects in the exercise group will receive prescribed exercise programs based on the individual's work capacity tests. All exercise sessions will be led by trained personnel, either graduate students or investigators.