Cytokines appear to play a critical role in the establishment of pregnancy. In this study, we measured serum and amniotic fluid concentrations of macrophage migration inhibitory factor (MIF) on the maternal hosts in murine pregnancy by enzyme-linked immunosorbent assay. The MIF concentrations in the murine maternal circulation in pregnancy ranged from 10 to 200 ng/ml and the level of MIF was lowest on Day 3 of pregnancy. On Day 4, the level of MIF recovered to the non-pregnant level and remained unchanged after implantation. MIF was also detected in the amniotic fluid, but the concentration was not parallel with that of MIF in the maternal circulation. MIF in the amniotic fluid, possibly produced by embryos, may have an autocrine function for embryo development. The temporary decrease of MIF in the maternal circulation may not directly relate to the development of embryos; however, it appears as an early sign of pregnancy for the maternal host.