Riddle and Braucher1 showed that injections of the hypophyseal lactogenic hormone, prolactin, discovered by Stricker and Grueter2, will cause growth of the crop glands of the pigeon. In view of the fact that established lactation is inhibited by administration of aestrogenic hormones3,4,5, it is of considerable interest to determine whether or not simultaneous administration of aestrogens will affect the response of the pigeon crop to prolactin injections.