Under chronic experimental treatment the rat thyroid shows two types of neoplasia, one follicular, apparently of epithelial origin, and the other solid arising presumably from parafollicular cells. It is assumed that the rat parafollicular cells represent embryonic ultimobranchial remains. On the other hand, the bird thyroid apparently does not contain this tissue since the ultimobranchial gland, in this species, is a separate organ. Three questions are posed; (1) Will chronic iodine deficiency in the bird result in both types of thyroid tumors being formed? (2) What effect does prolonged iodine deficiency and various levels of dietary calcium have on the thyroid of the rat, or the bird? (3) What effect does alteration of calcium and iodine intake have on the ultimobranchial gland of the bird? It is proposed to investigate the relationship of iodine and calcium intake on the formation and development of follicular and solid thyroid tumors in both the rat and bird. Two methods of inducing thyroid tumors, (1) Prolonged treatment with thiouracil, and (2) A low iodine diet-along with variable levels of dietary calcium - will be used. At intervals animals will be given radio-active iodine and killed 24 hours later. Following autopsy, histological and radio-autographic examination of the thyroid will be made as well as serum analysis of radioiodinated compounds. In the bird histological evaluation of the ultimobranchial glands will also be done.