Treatment of kidney encapsulated rats with 850 μC. of radioactive iodine effectively prevented both the characteristic rise of blood pressure to hypertensive levels and cardiac hypertrophy. Iodine-deficient diet was ineffective under these conditions. Once hypertension was established, treatment with 850 μC. radioactive iodine lowered blood pressure toward normal levels although a significant decrease in heart weight was not observed. The iodine-deficient diet was also ineffective under these conditions. It is felt that hypothyroidism produced by administration of high doses of radioactive iodine is as effective in preventing blood pressure rise as that observed with propylthiouracil treatment. The antihypertensive effect of these antithyroid treatments appears to be the direct result of inhibition of secretion of thyroid hormone.