Six nearly mature mongrel female puppies had the entire left hind limb immobilized in a plaster-of-Paris spica cast. After establishing the dose of porcine calcitonin that would significantly lower the concentrations of serum calcium and phosphate, this dose of the hormone (0.25 M.R.C. unit per kilogram of body weight) in a gelatin suspension was given subcutaneously every eight hours to three of the six animals its an attempt to prevent or to alter the pattern of bone loss that could be expected to develop in the immobilized limb after eight weeks. Even though calcitonin induced hypocalcemia and hypophosphatemia for fifteen to eighteen hours of every day, the bones of the animals treated with the hormone did not show any beneficial effects from its administration. Bone resorption, which was greatly increased in the immobilized limbs, was not inhibited by this dose of calcitonin in these animals. Since, in theory, a substance such as calcitonin could be useful in the treatment of local or generalized bone diseases characterized by excessive amounts of bone resorption, it is suggested that its possible efficacy is negated, at least in part, by the increased secretion of parathyroid hormone that occurs in response to the hypocalcemia produced by the administration of calcitonin to the intact animal.