Hepatocyte growth factor (HGF) is implicated in placental development; hgfr and hgf null mutant embryos develop placental insufficiency and lethality at 11.5 days (E11.5) after fertilization. The function of HGF in placentation at implantation (E4.5) has not been studied. Using reverse transcription-polymerase chain reaction, we detected HGF receptor (HGFR) mRNA in preimplantation embryos and in cultured blastocyst outgrowths. HGFR protein was detected in trophoblast cells in blastocyst outgrowths. HGF mRNA was not detected at these stages but was detected in the uterus at E5.5. Using in situ hybridization, we detected HGF mRNA in the mesometrial uterus, near the embryo, from E6.5 through E8.5. At E8.5, HGFR mRNA was detected in the chorionic placenta, and HGF mRNA was detected in the allantois. The expression for HGF and HGFR suggested a maternal-to-embryonic communication before the development of the allantois. To test this, blastocyst outgrowths were cultured with HGF. HGF stimulated the outgrowth of trophoblasts in a time-dependent manner and stimulated the expression of proliferating cell nuclear antigen, but it did not scatter trophoblasts. HGF stimulated an increase in the trophoblast cell number, but caused a decrease in the total number of terminally differentiated trophoblasts expressing placental lactogen-1 protein. These data suggest that HGF stimulates the cell division, but not the differentiation, of trophoblast cells during implantation.