Yearling heifers overfed protein, calcium and phosphorus with a feed recommended for high producing dairy cows developed osteopetrosis and skeletal malformations as a result of retarded bone resorption. Histologic and electron microscopic examinations showed that C cell hyperplasia was also present. The presence of C cell hyperplasia and osteopetrosis supported the diagnosis of hypercalcitoninism. Clinically unaffected heifers were studied by serum chemistry after 1 to 1.5 months on the same diet at age 6 months and after 3 months on an optimal diet to learn whether they showed evidence of hypercalcitoninism as expressed by retarded bone resorption and/or hypercalcitoninemia. The data indicated that bone resorption was retarded and that serum gastrin was elevated in the heifers without skeletal malformations while on the high calcium feed. The heifers were isocalcitoninemic. The data suggested that overfeeding calcium was sufficient to produce hypercalcitoninism but the rate of calcitonin secretion had not exceeded the rate of removal of calcitonin from blood by binding to tissue receptors and by metabolic degradation. Since calcitonin is rapidly removed from blood, hypercalcitoninism may occur without concomitant hypercalcitoninemia.