During pregnancy, mouse uterine epithelial cells produce and secrete a large amount of macrophage colony-stimulating factor (M-CSF/CSF-1). Macrophages accumulate and proliferate in the undecidualized endometrium of the pregnant uterus. Observations showed that macrophages expressed scavenger receptor class A (type I and type II) and class C (macrosialin). Scavenger receptors appeared to be involved in the removal of apoptotic cells in the degenerated decidual tissue. The expression of class A and class C scavenger receptor mRNAs in the uterus of pregnant mice was elevated but the expression of class B scavenger receptor (CD36) mRNA was similar to that of non-pregnant mice. The expression of various cytokines and chemokines, including M-CSF, monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein 1-alpha (MIP1-alpha), was enhanced in the uterus of pregnant mice, suggesting that these molecules regulate macrophage chemotaxis and immunological function in the uterus. These findings imply that the pregnant uterus provides a microenvironment for the recruitment, differentiation, and proliferation of macrophages and the regulation of scavenger receptor and cytokine expression for a successful pregnancy.