Using rats previously labeled with 45Ca, the effects of a severely phosphate deficient diet on calcium mobilization from bone into serum were examined in both intact and thyroparathyroidectomized (TPTX) RATS. With the TPTX animals, increased calcium mobilization from bone was evident 12 hr after the rats had been placed on the low phosphorus diet. At that time period, both TPTX and intact rats had become severely hypophosphatemic. However, in intact rats, calcium mobilization was not observed until 48 hr had elapsed. Both intact and TPTX hypophosphatemic rats developed hypercalcemia. To determine if inhibition of calcium deposition into bone contributed to this change, the course of 45Ca movement from blood into bone was followed in an experiment where rats received a single injection of the isotope at the time the low phosphorus diet was given. The animals on the low phosphorus diet showed a significantly lower bone specific activity and a higher serum specific activity compared to the control group, indicating calcium deposition into bone was inhibited. We conclude that the acute response to hypophosphatemia, resulting from the low phosphorus dietary regimen, was an increase in bone resorption and an inhibition of bone mineralization. The increase in bone resorption occurred more rapidly in TPTX rats than in the intact animals.