The studies in this dissertation were designed to investigate the immunomodulatory changes that occur in the cytotoxic lymphocyte populations of the horse at the systemic and regional levels during early pregnancy. Pregnant mares demonstrate a reduction in cytotoxic T lymphocyte (CTL) reactivity against target cells from the breeding stallion. I investigated whether this effect is limited to activity against paternal Major Histocompatibility Complex (MHC) antigens, and whether it occurs during MHC compatible pregnancy. Mares carrying MHC incompatible pregnancies demonstrated reduced CTL activity against lymphocytes from horses with unrelated MHC haplotypes in addition to those from the breeding stallion. This effect was also observed in mares carrying MHC compatible pregnancies, suggesting that an antigen-independent mechanism may be responsible for the decrease in CTL reactivity. I then examined whether natural killer (NK) cells may be present in the uterus among the dense lymphocytic infiltrate that surrounds the placental trophoblasts of the endometrial cups. Using quantitative molecular techniques, I identified the equine homologs of the NK cell marker genes NKP46, CD16, CD56, and CD94 and found that gene expression of all four markers was significantly higher in lymphocytes isolated from the endometrial cups compared to peripheral blood lymphocytes (PBL) isolated from the same animal on the same day. This provides the first evidence for the existence of NK cells in the equine endometrium during pregnancy. After developing panels of monoclonal antibodies that specifically recognize the equine homologs of NKp46 and CD16, I performed experiments to characterize equine NK cells. The two markers labeled a small percentage of PBL, and were also expressed on a subpopulation of CD3+ cells in most horses, suggesting that NK cells in the horse may have a CD3+ NK cell phenotype that differs from other species, or that the primary cell type recognized by these markers is the Natural Killer T cell. Immunohistochemical labelling of endometrial cup tissue sections showed that endometrial cup lymphocytes (ECL) consisted of predominantly CD16+ cells, which overlapped with a previously observed CD3+ population. This novel finding suggests that the CD3+ ECL population is also CD16+, and represents a dramatic increase over the small peripheral CD16+ population. Overall, this study advances knowledge of changes in maternal immunity during pregnancy in the mare and provides a more complete characterization of uterine leukocytes in the horse.