Diversity of opinion exists regarding the amount of estrogen necessary to produce the interval phase of the endometrium in castrate women. Some clinical investigators1believe that the administration of from 30,000 to 50,000 rat units is needed to obtain this result. The experiments of Werner and Collier2in which thirteen castrate women were given intramuscular injections of theelin in aqueous solution showed that the growth phase could be produced by much smaller dosages than those just indicated and that approximately 2,800 rat units would initiate endometrial growth. We believed that it would be of value to repeat this work, especially since theelin in oil was available. An investigation was decided on which included a series of separate experiments to run concurrently. Among the points of interest to be determined were (1) the dosages of theelin in oil that will relieve symptoms resulting from castration, and the relation of