Seven hyperphosphatemic hypoparathyroid adult subject gave informed consent to participate in the 72-hour study during which there was an 8 hour intravenous infusion of calcium chloride followed by a 16 hour washout. During the study serial blood samples were collected for chemical analysis. In four subjects there was a significant increase (up to 200%) in serum FGF-23 within the first 24 hours following the calcium chloride infusion during which the serum ionized calcium increased (mean difference 0.38 mmol/L) and serum ionized magnesium decreased ( mean difference 0.09 mmol/L). In addition, there was also a significant (approximately 25%) increase in serum phosphorus. In three subjects that were pre-treated with PTH the infusion of calcium chloride did not affect the serum P and FGF-23 concentrations. These preliminary results indicate that the serum FGF-23 concentration is regulated by changes in serum P and not vitamin 1,25-D.