Regulation of plasma free calcium concentration displays the following features: (1) The level in an individual varies within fairly narrow limits and is characterized by a mean (an estimate of the individual set point) and a standard deviation (an estimate of oscillation about the set point). (2) There is an intrinsic circadian oscillation that is independent of external disturbing signals. (3) Individual values (set points) are partly under genetic control with significant between-family differences. (4) Correction of errors is rather slow-it usually takes at least 12 hours to restore the basal level after experimental induction of hyper- or hypocalcemia. (5) Correction of errors can occur with normal efficiency in the absence of parathyroid hormone or calcitonin, provided the system is adequately primed with some form of vitamin D. These and other facts are best explained by a concept that locates the calcium homeostatic system at the quiescent bone surface. Across this cellular and tissue barrier exchange of calcium between blood and bone in the absence of net flux can be demonstrated by autoradiography. The distinction between the homeostatic system and the remodeling system that regulates bone mass is not absolute, since the latter can participate in short term correction of errors, and if the obligatory calcium loss (determined by the difference between the plasma calcium set point and the threshold for renal tubular reabsorption of calcium) is too high to be satisfied by net intestinal absorption, the remodeling system must make up the difference by sacrificing bone.(ABSTRACT TRUNCATED AT 250 WORDS)