The decidua basalis of developing mouse implantation sites is highly enriched in CD45+ leukocytes. In intact, syngeneically mated C57BL/6 decidua basalis examined at gestation day 8.5 by whole-mount in situ immunohistochemistry, leukocyte, but not trophoblast, conjugations were reported. Nothing is known regarding time course, frequency, composition, or importance of physiologic decidual CD45+ cell pairing. In this study, we confirmed the presence of anti-CD54+/anti-CD11a+ immune synapses in CD45+ decidual cell conjugates and characterized their cellular heterogeneity. Conjugated cell pairs were virtually absent before implantation (virgin and gestation days 3.5 and 4.5), were infrequent at gestation day 5.5, but involved 19% of all CD45+ cells by gestation day 8.5, then declined. By gestation day 8.5, almost all CD45+ cells coexpressed CD31, and 2 CD45+CD31+ cells composed most conjugates. Conjugation partners were defined for 2 nonoverlapping uterine natural killer cell subsets (Ly49C/I +/Dolichos biflorus agglutinin lectin- and Ly49C/I-/Dolichos biflorus agglutinin lectin+). Ly49C/I+ uterine natural killer cells were the major subset from before mating up to gestation day 6.5. At gestation day 5.5/6.5, uterine natural killer cell conjugates involving Ly49C/I + cells were more abundant. By gestation day 8.5/9.5, Dolichos biflorus agglutinin lectin+ uterine natural killer cells were the dominant subset with Dolichos biflorus agglutinin lectin+/Dolichos biflorus agglutinin lectin+ homologous conjugates and Dolichos biflorus agglutinin lectin+/Dolichos biflorus agglutinin lectin- heterologous conjugates dominating uterine natural killer cell pairings. At gestation day 6.5, both Ly49C/I+/CD45+ and Dolichos biflorus agglutinin lectin+/CD45+ heterologous conjugate pairs strongly engaged antigen-presenting cells (CD11c+, CD68+, or major histocompatibility complex class II+). By gestation day 8.5, dominant partners of Ly49C/I+/CD45+ and Dolichos biflorus agglutinin lectin+/CD45+ heterologous conjugates are T cells (CD8+ >CD4+). Heterologous conjugates that did not involve uterine natural killer cells occurred but did not suggest antigen presentation to T cells. These data identify gestation day 6.5-8.5 in the pregnant mouse as a critical window for leukocyte interactions that may establish immune regulation within implantation sites.