Osteoporosis causes bones to weaken and break more easily. Alendronate is used to treat or prevent osteoporosis. PTH is a protein hormone that increases the calcium and phosphorus release from bone, leading to formation of new bone. This study will examine the changes in bone density measured at multiple places in the skeleton and changes in chemicals in the body that indicate bone breakdown and bone formation. The study will indicate whether some breakdown of bone is required for PTH to have an overall bone-building effect in men.