ESTROGEN treatment has been variously reported to increase thyroid activity (Emge and Laqueur, 1941; Reineke and Soliman, 1953; Engstrom and Markardt, 1954), to have little effect on thyroid function (Paschkis, Cantarow and Peacock, 1948; Desclin and Ermans, 1951), and to inhibit thyroid cell growth and colloid resorption (Gardner, 1949; Tuchman-Duplessis and Mercier-Parot, 1953). Underfeeding reduces the rat thyroid response to thiouracil in proportion to the reduction in body growth (Meites and Agrawala, 1949). Meites (1949) showed that large doses of estrogen will reduce the appetite of rats and speculated as to whether the decrease in food intake was the cause of, or a result of, an inhibition of thyrotrophin secretion by the estrogen.