Abstract
The accretion rate and size of the exchangeable mineral pools of tibiae of mice treated with estrone and parathyroid extract have been measured employing Ca45 and Na22. Estrone was found to accelerate the rate of calcium accretion and to enlarge the size of the labile bone mineral pool. Parathyroid extract diminished slightly the accretion rate but had not effect upon calcium exchange. When both hormones were administered simultaneously an antagonism between them was observed that was not a function of ion exchange phenomena but presumably was dependent on changes of metabolic processes associated with the formation of new bone crystal and the growth of established crystal.