Mouse and vole embryos were allogeneically and xenogeneically transferred into pseudopregnant CD.1 and immunodeficient (seid)female mice,and we investigated the distribution of uterine leucocytes cells in the implantation sites on days 5,6,and 7 of pregnancy. Maerophages were evenly distributed in the endometrium on days 5-7.Neutrophils were rarely seen on days 5-7,but lymphocytes were found throughout the endometrium,often in groups associated with glands or the luminal epithelium.The number of uNK cells increased markedly at the mesometrial uriangle and the outer decidual area in the CD-1 uteri containing vole embryos;by contrast,seid uteri having vole embryos showed almost the same number as those having mouse embryos.Mast cells were present in large numbers at the myometrium,but rarely in the decidua in all types of pregnant uteri.Cells at the myometrium were more numerous in xenogeneic than in allogeneic transfer.Maay mast cells appeared in the inner decidua where xenogeneically transferred vole embryos were dead and aborted.These results suggest the possibility that uterine leucocytes mediate various immunological events in the mouse-vole interspesific pregnancies.