Pseudopregnancy, induced by mating females with vasectomized males, is a frequently used model for studying pregnancy-like uterine changes in the absence of an embryo. Leukocytes make a significant contribution to uterine cellularity during pregnancy. The present study was designed to determine whether changes in numbers and distribution of leukocytes in the uterus during pseudopregnancy and following intraluminal injection of a deciduogenic stimulus parallel changes observed during the first eight days of pregnancy. Common leukocyte antigen positive (CLA+) cells, macrophages (F4/80+ cells), and granulocytes were assessed between days 1 and 8 of pseudopregnancy using qualitative and quantitative immunohistochemistry. High numbers of CLA+ leukocytes were present on days 1 and 2. Those were comprised primarily by macrophages, polymorphonuclear leukocytes, and eosinophils. High concentrations of leukocytes were detected in the endometrium, and some granulocytes were observed migrating through the luminal epithelium. Leukocytes, principally macrophages, were reduced in number and were distributed throughout the endometrium on days 3 and 4. Introduction of oil to the uterine lumen on day 4 stimulated primary decidualization. Decidual cells were CLA- and F4/80-, and, as decidualization proceeded, CLA+ and F4/80+ cells decreased in number in the anti-mesometrial uterus and were detected primarily in the deep endometrium. Later, a secondary decidualization zone developed in the mesometrial aspect of the uterus. Unlike the initial decidual reaction, which was relatively free of leukocytes, the secondary decidual zone contained very high numbers of CLA+ and F4/80+ cells. The uninjected uterine horn remained relatively unchanged from days 3 through 8.(ABSTRACT TRUNCATED AT 250 WORDS)