The mechanisms of phosphate and calcium homeostasis in senile osteoporosis and osteomalacia were studied. Findings include: (1) 1,25-dihydroxy vitamin D3, incubated in vitro with isolated intestinal cells, stimulated calcium uptake, the increase being specific for the 1,25-dyhydroxy metabolite and blocked by inhibitors of protein synthesis; 2) calcium was transported by renal tubule basolateral membrane vesicles by a sodium/calcium exchange mechanism, the system beign regulated by parathyroid hormone; (3) the establishment of the isolated renal cell as a model for studies on the regulation of the hydroxylation of 25-hydroxy vitamin D3; (4) the further characterization of the adaptation in renal phosphate transport in response to dietary phosphorus; (5) the characterization of phosphate uptake in a cultured renal cell line and the demonstration that the uptake is controlled by parathyroid hormone.