The effect of monosodium glutamate (MSG) on the thyroid gland of mice is studied. MSG induces marked histological changes in the thyroid tissue, indicative of hypothyroidism. The follicular epithelium, is greatly compressed and the follicles have enlarged due to distension with the accumulation of colloid. The histological picture of the thyroid gland remains unchanged for the animals sacrificed after short-term (13 weeks) and long-term (52 weeks) treatment. MSG, therefore, has a cumulative effect. The hypothyroidism is attributed to the influence of the drug on hypothalamus-pituitary function and the secretion of hormones responsible for the thyroid metabolism.