Traumatization of uteri of rats with ovaries luteinized by the PMS-HCG priming procedure of Parlow fails to elicit formation of deciduomata when performed 2, 3, 4, 5, 6 or 7 days after HCG treatment. Since progesterone synthesis and secretion by such ovaries has been demonstrated, it is concluded that the lack of deciduomal response is due to secretion of excessive amounts of an inhibitor of this response. Neither prolactin nor luteinizing hormone overcomes the failure of the deciduomal response. Evidence of increased estrogen secretion (uterine enlargement and edema, increased stratification and mucification of vaginal epithelium) follows treatment of PMS-HCG primed rats with luteinizing hormone. Equine LH produces such effects whether administered in an aqueous vehicle twice daily, once daily, or once at the beginning of a 5-day period. Ovine LH is effective if administered in an aqueous vehicle twice daily, but not if administered less frequently. Bovine LH is ineffective even when given twice daily in an aqueous vehicle. When absorption of bovine or ovine LH is delayed by injecting these preparations as suspensions in 5 % beeswax in sesame oil, their effectiveness in eliciting signs of estrogen secretion is greatly enhanced. (Endocrinology76: 246, 1965)