Six experiments were conducted to determine the mechanism of action of exogenous PGF2alpha on the clearance of uterine infections in sheep and pigs. The first two experiments were designed to characterize the uterine immune response to bacterial infection under progesterone dominance in pigs. The uterine immune response to infections seems to change with parity. This is probably an artifact of increased number of bacterial exposures; therefore, the third experiment was designed to evaluate the uterine immune response to multiple intrauterine bacterial inoculations. Experiments 4, 5, and 6 were designed to evaluate the effects of endogenous and exogenous PGF2alpha on the uterine immune response to uterine infections in sheep and pigs. Injections with Lutalyse (PGF2alpha analogue) during the luteal phase in sheep causes luteolysis; therefore, it impossible to evaluate the effects of Lutalyse independently of luteolysis. In order to cause an endogenous release of PGF2alpha without causing luteolysis in sheep a PGF2alpha secretagogue (oxytocin) was used in Exp. 5. And in Exp. 6, we were able to evaluate the effects of Lutalyse independently of luteolysis using pigs as a model. From these six experiments we concluded that during periods of estrogen dominance, the uterine immune system is up-regulated, and therefore, infections do not develop after intrauterine inoculation with bacteria, during periods of progesterone dominance, the uterine immune system is down-regulated, and, therefore, infections develop after intrauterine inoculation with bacteria, and stimulation of the uterus with PGF2alpha or oxytocin independently of luteolysis up-regulates the uterine immune. / Ph. D.