SUMMARY A technique for the rapid hypophysectomy of immature female mice is described. The response of the uterus to pituitary and placental gonadotrophins is compared in normal mice, mice hypophysectomized at the time of injection, and 24 hr prior to injection. The results suggest that responses to placental gonadotrophins are limited by endogenous gonadotrophin production, while those to impure pituitary gonadotrophins are not.