Uterine Natural Killer (NK) cell numbers expand after ovulation and persist into early pregnancy, suggesting roles in implantation/placentation. Adhesion of peripheral blood NK (PBNK) cells to uterine vascular endothelium is enhanced around ovulation, potentially promoting migration to the uterus. The goals of this thesis are to examine endocrine regulation of endothelial adherence of PBNK cells from fertile women and embryo transfer patients during in vitro fertilization cycles or monitored, non-medicated menstrual cycles. Fluorescently labeled PBNK cells from serial blood samples were applied to pregnant mouse uterine tissues. PBNK cell adhesion peaked at ovulation in fertile women and those establishing pregnancy. Adhesion patterns differed for non-pregnant subjects. Microarray analysis of PBNK cells from fertile women at day 5 and LH surge revealed significant up-regulation for chemokine receptor CXCR4 and TNF[alpha] protein 3. These data are the first to demonstrate that menstrual cycle hormones functionally alter the trafficking potential of blood NK cells.