1. Two hundred male Long-Evans rats, weaned at twenty-one days of age, were injected daily with stable strontium for twenty-one days. Stable strontium was then discontinued and approximately half of the total number of rats received a single sublethal injection of Sr⁹⁰. The metabolism of Sr⁹⁰, the effects of radiation damage on the mineral metabolism of bone, and the effects of phosphorus-deficient diets on the mobilization of both stable strontium and Sr⁹⁰ from bone were studied for fifty-four to sixty-four days after injection.

2. After a single injection of Sr⁹⁰, the initial rapid rise in plasma activity is succeeded by a rapid decline. An attempt was made to correlate the changes in plasma activity with the rates of excretion and of skeletal uptake of the isotope.

3. Mobilization of stable strontium from bone was delayed in rats that received Sr⁹⁰.

4. Diets deficient in phosphorus became effective in increasing the mobilization of both stable strontium and Sr⁹⁰ from bone seven days after the diets were instituted. However, thirty days after the administration of the isotope the diets became relatively ineffective in mobilizing mineral in the Sr⁹⁰-injected rats.

5. It appears that the effectiveness of phosphorus-deficient diets in accelerating bone resorption is dependent on the presence of an adequate number of viable bone cells and adequate circulation to the bone, both of which may be affected by exposure to radiation at the dosages of Sr⁹⁰ employed in this experiment.