Following the prolonged administration of synthetic oestrogens to rats it has been shown that changes occur which are associated with a decreased function of the anterior pituitary gland [Noble, 1938a, 1938b, 1939]. When crystals or tablets of active substances were implanted into the subcutaneous tissues it was found that although they produced the typical changes the amount absorbed was very great over a prolonged period. Since inhibition of body-growth, lactation, and gonadotrophic hormone secretion of the anterior pituitary gland is readily produced by oestrogens, it was thought to be of importance to determine quantitatively the amount of oestrogen necessary to produce these effects. The relatively high oestrogenic activity of diethylstilboestrol when administered orally [Dodds, Lawson, and Noble, 1938; Parkes, Dodds, and Noble, 1938; Emmens, 1939], and its comparatively high solubility in aqueous solution, suggested that the oral administration of aqueous solutions could be adopted. By the substitution of