The influence of estradiol on the radioactive calcium metabolism in blood, bone and young of lactating rats has been studied. Estrogen, in an amount estimated to equal that secreted in late pregnancy, was injected daily into lactating rats for 4 days along with 45Ca. On the 5th day, radioactivity of blood serum, bone, and young was compared to a similar group of lactating rats not treated with estrogen. Average gain in weight of the litters of the 2 groups was similar. However, radioactivity in serum and bones of treated with estradiol was significantly higher than in the controls. The radioactivity in litters of the 2 groups was similar. Estrogen plays a role in the retention of calcium in the animal body.