The thyroid glands from 385 rats have been studied. Diffuse hyperplasia was produced in rats through the feeding of the Steenbock rachitogenic diet or its various modifications. Hyperplasia developed in the presence or absence of the antirachitic vitamin. The factors underlying the development of hyperplasia in experimental animals on these diets were a deficiency of iodine, associated with an excessive amount of calcium carbonate. Increasing the amount of calcium carbonate in diets deficient in iodine resulted in enhancement of the degree of hyperplasia. Variations in the degree of hyperplasia were found within similar experimental groups. The addition of a small amount of potassium iodide to the diets prevented the development of hyperplasia. A high incidence of goitre was found in a series of wild rats from this city.