Administration of specific and wellcharacterized rabbit antisera to ovine luteinizing hormone (LH) to pregnant rats, before day 12 of gestation, resulted in vaginal bleeding followed by resorption of fetuses. This effect of the antiserum could not be reversed by the administration of either ovine prolactin or extracts of day 12 rat placentae. Homotransplantation of 2 pituitaries under the kidney capsule also did not reverse this effect. Whereas 0.25–5 μg estradiol-17β was ineffective, 4 mg of progesterone could overcome the effects of the antiserum. Deprivation of LH even for 2–8 hr was injurious to fetal growth. These effects of the antiserum could be reversed by the simultaneous administration of LH. Thus, evidence has been presented to show that LH maintains pregnancy up to day 12 by stimulating steroidogenesis. Of the 3 gonadotropins (LH, prolactin and FSH) tried, administration of LH during the second half of pregnancy resulted in postponement of parturition. Similar results were obtained with the administration of progesterone but not with 20α-hydroxypregn- 4-en-3-one (20α-OH-P) or estradiol. Based on these results, an integrated model for the hormonal control of pregnancy in the intact rat has been presented and the role of LH and prolactin in this process has been discussed. (Endocrinology86: 874, 1970)