This report describes studies on the mechanisms of phosphate and calcium homeostasis relevant to changes in mineral metabolism during the aging process.  The findings relate to investigations on: (a) Age-related phosphaturia and hypophosphatemia. ( b) Renal adaptation to phosphate load in the acutely thyroparathyroidectomized rat: rapid alteration in brush border membrane phosphate transport. ( c) Phosphate reabsorption in progressive renal failure: effects of parathyroidectomy and phosphate restriction. ( d) Phosphate uptake in cultured renal cells: mechanisms of hormonal regulation. ( e) The physiological mechanism by which 1,25-dihydroxycholecalciferol increases intestinal calcium absorption. ( f) The biochemical mechanisms by which 1,25-dihydroxycholecalciferol increases intestinal calcium absorption. ( g) Age-dependent changes in the metabolism of vitamin D. (h) Regulation of intestinal calcium absorption by diet. ( i) Renal calcium transport, sodium/calcium exchange in renal tubule baso-lateral membranes and the action of parathyroid hormone.