Hyperplastic goiters were observed frequently in female rats of Wistar strain when given defatted soybean under iodine deficiency for 6 to 12 months. The findings in the thyroid were those of malignant tumors in which the features of thyroid carcinomas were seen, accompanied with metastasis in the lungs of some animals. Enlargement of the thyroid was completely inhibited in rats when a small amount of iodine was added to the diet. The role of soybean factor(s) which causes enlargment of the thyroid is discussed in relation to the development of malignant goiter, together with pathological findings in the animals.