The spatiotemporal distribution of macrophages in the uterine wall of virgin mice and mice in the first half of pregnancy has been studied. Macrophages were identified using a combination of morphological criteria, the capacity to endocytose horseradish peroxidase and the expression of the Mac-1 antigen. In virgin mice and mice at preimplantation stages of pregnancy, macrophages were found throughout the endometrium, myometrium and mesometrial triangle. Following implantation, and in parallel with decidualisation, the density of macrophages appeared to decline in the decidua with advancing gestation. It is suggested that this change in density is due to a dilution of the macrophage population rather than a loss of individual cells. The numbers and distribution of decidual macrophages indicate that this group of cells does not play a major regulatory role in the success of pregnancy.