Successful induction of decidualization on the 4th day of pseudopregnancy (L4) is possible in rats hypophysectomized on L3 of pseudopregnancy (after 19.00 hours). However, in the absence of exogenous hormone, the decidual cell reaction cannot be maintained in the hypophysectomized rat. The effect of hypophysectomy on decidualization which is already established depends upon the time of removal of the pituitary. When hypophysectomy was performed before the 11th day of pseudopregnancy (L11) breakdown and resorption of the deciduomata occurred within 4 days; when hypophysectomy was carried out on L11 or L12, the deciduomata were resorbed more slowly. This relatively prolonged resorption was related to the absence of ovulation at the end of the period of pseudopregnancy in the hypophysectomized rats. The results indicate that no luteotrophic factor of uterine or decidual origin is produced in the pseudopregnant rat bearing deciduomata.