SummaryDeladroxone, a long-acting progestogen, had no effect upon the whole body retention of 85Sr in young rats and mice. Pretreatment of rats with Deladroxate, which contains Deladroxone plus estradiol enanthate, reduced by nearly 50% the 21-day retention of 85Sr. However, when administered to mice in large doses after the 85Sr had been well fixed in bone, Deladroxate raised the retention 8-9%. The effects of Deladroxate in both species appeared to be due to its estrogen component.