Survival rates have been studied in parathyroidectomized rats that were placed on a low calcium diet either 4 days before, immediately after, or 7 days after, the operation. In addition, serum calcium and phosphorus levels were determined for periods up to 2 months after parathyroidectomy in rats maintained continually on a normal diet. Of rats maintained on a normal diet, 79 % survived the first 7 days after parathyroidectomy. When these rats were then placed on a low calcium diet for 5 days, 74 % of the original number of rats survived this stress. In sharp contrast with this high survival rate resulting from the imposition of a low calcium diet 7 days after parathyroidectomy were the results obtained by imposing a low calcium diet 4 days before parathyroidectomy; under the latter conditions only 8% survived the first 24 hours after the operation. The functional development of accessory parathyroid tissue during the postparathyroidectomy period, a logical explanation of the survival data, did not seem to be a factor. Possible accessory tissue in the thymus was ruled out as being functionally involved. The majority of the serum calcium and phosphorus values remained at hypoparathyroid levels for as long as 2 months after parathyroidectomy, a fact that did not support the idea of widespread accessory tissue development, either of thymic or of extrathymic location. A small number (11 %) of the serum calcium values did return to normal levels (>9.0 mg/ 100 ml), implying that in these rats accessory tissue was functional. No clear-cut explanation of the survival data can be given, but several possibilities are discussed.