To examine the effects of PTH (parathyroid hormone) on calcium metabolism, subjects will be admitted to the GCRC for 3 days in the beginning of the study. Subjects will have a plastic needle inserted into a vein to enable the nurses to draw your blood repeatedly without having to stick you every time. Subjects will have 120 mL (about 8 tablespoons) of blood drawn during the 3 day period. Subjects will be allowed to choose the food from the regular hospital menu but the amount of calcium from the diet will be the same during each day that you spend in the GCRC. The first day will be used to observe calcium balance without medication while the subsequent 2 days will be used to determine how calcium balance changes in response to the medication rhPTH. Subjects will take the medication in the morning, before breakfast - this will be considered time 0. During all 3 days, blood will be taken at 0, 2, 4, 6, 8, 12, 16, and 20 hours for examination of serum calcium and at 0, 4 and 12 hours for measurement of PTH (that is produced by your body).