This report describes a laboratory model designed to illustrate a newer concept for the control of plasma calcium concentrations. This postulate suggests that plasma calcium levels are the result of a balance or imbalance in opposing calcium ion fluxes between plasma and bone fluid compartments existing around osteocyte-lining cells (osteoblasts) units in bone. The metabolic control of these fluxes is postulated to reside within the lining cells on the surface of bone. These cells serve as a cellular interface between the two fluid compartments. The model described in this report illustrates these principles by using the height of columns of water in cylinders to represent calcium ion concentrations. A water pump, representing the calcium transcellular transport system in the lining cells, maintains a higher level of water in the cylinder representing plasma than in that representing bone fluid. This is accomplished by continuous pumping of water from the bone fluid cylinder to the plasma cylinder. Water is returned to the bone fluid cylinder as long as a differential in the height of water in the two cylinders exists. A constant height of water in the plasma cylinder is maintained when the two fluxes are in equilibrium. A constant height of water in the cylinder representing bone fluid is maintained by the level of water in a much larger cylinder representing calcium equilibrium between the solid and liquid phases of bone. The primary hormone controlling the rate of calcium transfer from bone fluid to blood, or in the model regulating the speed of water pump, is parathyroid hormone.