Treatment of the immature animal with æstrogenic or gonadotropic substances has been described by a number of authors in diverse experiments, and vaginal cornification, enlargement and typical histological changes in the uterus, ovulation, mating, and fertilisation, have all been produced, but no successful implantation. The histological changes which occur in the uterus and vagina of the treated immature animal are considered (Reynolds and others) to be characteristic of heat and similar to those in the adult uterus during heat. Nevertheless, it is suggested that the failure to procure implantation in treated immature animals may be due to an unfavourable condition of the uterine endometrium. If that is so, there must exist some difference between the response of the treated immature uterus and that of the mature uterus during cestrus which has thus far escaped detection. Recently we have described (Bacsich and Wyburn, 1940b) a cyclic vascular change in the uterus of the guinea-pig in the form of antimesometric hyperæmia present only during normal heat, and, moreover, have been able to induce this vascular response in spayed animals by hormonal treatment (Bacsich and Wyburn, 1941). As this cyclic vascular change provides a particularly delicate test of a complete response of the uterus to ovarian hormonal activity and may influence the site and the course of implantation, it seemed of interest to ascertain if the “characteristic” œstrogenic response of the immature uterus included antimesometric hyperæmia.