This report is a study of the relative removal of radiocalcium and stable calcium by continuous peritoneal lavage in the rat, as influenced by the hormone of the parathyroids. The lavage procedure involved the use of male rats weighing between 225 and 275 gm which had been maintained 48 hours on a calcium free diet and had been nephrectomized 24 hours before experimental use. Parathyroidectomy was performed without interruption of the lavage procedure. The results indicated that if the radiocalcium was injected into the animals less than 24 hours before the lavage treatment, the radioactivity removed by this method was not affected by parathyroidectomy. This was in spite of the fact that the operation caused an immediate drop in the total calcium concentrations of the lavage washes. If, however, the radiocalcium was injected two to three weeks before the continuous lavage was performed, the radiocalcium concentrations of the wash parallel those of the stable calcium, both falling after parathyroidectomy. These data are construed to indicate that a basic equilibrium exists between the extracellular spaces and those portions of the bone with which the fluid is in ready contact, and that this equilibrium maintains the constant low concentration of calcium in the fluid spaces. It is further postulated that, in addition to this basic equilibrium, parathyroid hormone supplies calcium to the fluid spaces from areas of bone that are not in immediate contact with the surrounding fluid media, and that this is done through metabolic processes in bone cells which enables calcium to be dissolved in concentrations greater than the solubility of apatite crystals.