The role of LH in the leuteotrophic process was examined by evaluating progesterone secretion in pregnant, lactating, and pregnant lactating rats, and the response of these rats to antisera against LH (LHAS) was observed. Ovarian venous progesterone concentration in lactating rats was elevated as litter size increased. Treatment with LHAS on day 8 postpartum of rats nursing 12 pups significantly decreased peripheral progesterone levels within 24 hr. Treatment with LHAS on days 8 and 9 of pregnancy terminated gestation in all normal pregnant animals by in similarly treated pregnant lactating rats gestation was maintained until day 14 in 4 of 10 rats. If the litter was removed from pregnant lactating dams at the time of initiation of LHAS treatment, pregnancy was interrupted in all 12 animals. Ovarian venous progesterone concentration was reduced by greater than 90% 24 hr after LHAS treatment on day 8 of normal pregnant rats and 20alpha-OH-P increased such that the concentration of total progestins remained constant. Ovarian venous progesterone concentration 24 hr after LHAS treatment of pregnant lactating rats was reduced by 50% in animals receiving daily estradiol treatment by LHAS had little effect in pregnant lactating animals which received a single injection of estradiol on day 4 postpartum. The level of 20alpha-OH-P in pregnant lactating animals was not affected by LHAS. The results of these studies demonstrate that in lactating rats nursing a large litter (12 pups) LH is part of the luteotrophic complex, and the data obtained from studies in pregnant lactating rats suggest that when pregnancy and lactation are superimposed the role of LH in the luteotrophic process on day 8 of gestation is diminished.