['We evaluated implantation-associated quantitative changes in endometrial and peripheral natural killer (NK)-cell populations of pigs.', 'Natural killer cell populations were investigated in 10, 15, 20, 30 and 40 days pregnant and non-pregnant (NP) sows by flow cytometry, immunohistochemistry and morphometry.', 'The number of endometrial CD16(+) NK cells significantly declined at attachment phase of implantation and remained relatively low over the course of implantation. The CD16(+) NK cells in situ showed implantation-phase dependent density and localization. Prior to implantation, they substantially resided in the subepithelial stroma. As implantation advances, the density of NK cells into subepithelial stroma decreased while that of NK cells into glandular layer increased, suggesting implantation-induced re-location far from the attached conceptus. The number of CD56(+) lymphocytes was the greatest at pre-attachment phase of implantation, dropped at the time of attachment and increased up to end of early pregnancy period. The CD3(-) CD8(+) NK-cell number decreased significantly when the definitive placenta is established. No significant differences in the numbers of peripheral blood CD16(+), CD56(+) and CD3(-) CD8(+) NK cells between pregnant and NP animals as well as relative to the implantation phase were observed.'] Is superficial implantation in pigs associated with decreased numbers and redistribution of endometrial NK-cell populations?