The thyrocalcitonin content of the thyroid glands of euthyroid, hyperthyroid and hypothyroid rats fed a calcium deficient diet was determined in 2 separate experiments by bioassay. Hyperthyroidism was produced by the injection of 50 μg l-thyroxine or 200 mU thyrotropin per day, and hypothyroidism by the dietary administration of 0.05% propylthiouracil or an iodine deficient diet. The log-dose response curve for the hypocalcemic activity of the thyroid homogenates from the euthyroid rats was linear between 0.24 and 0.96 mg thyroid tissue, equivalent to 0.018–0.072 of a single rat thyroid gland. No significant differences in the hypocalcemic response of assay rats was observed when a comparison was made of thyroid tissue homogenates that included parathyroid tissue and thyroid homogenates in which the parathyroid gland was removed prior to tissue homogenization. Thyroxine consistently caused a significant 27–35% reduction in the thyroidal content of thyrocalcitonin, whereas the administration of thyrotropin...