1. Parathyroid hormone (PTH) injected intravenously into rats caused a small rapid uptake of simultaneously administered radioactive calcium into the skeleton.
2. This skeletal uptake of calcium occurs in many other species, including the monkey and probably man. It appears to be an initiating event in PTH action on bone.
3. Although calcitonin prevents PTH-induced bone resorption, it did not affect this early calcium shift.
4. This evidence is discussed in connection with the possibility of combining calcitonin with some other agent for the treatment of osteoporosis.