Expression of vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) and its receptors (flt-1 and flk-1) during the peri-implantation period (days 3, 4, 5, 6 and 7 post coitus) in the golden hamster was investigated by in situ hybridization, immunohistochemistry and the reverse transcription/polymerase chain reaction (RT-PCR). Three days after mating, in situ hybridization and immunohistochemical staining revealed weak VEGF expression only in the uterine epithelium; this expression was similar to that seen at oestrus. Flt-1 but no flk-1 immunoreactivity was observed. At day 4, the subepithelial stroma and embryo displayed immunoreactivity for VEGF and flt-1, whereas endothelial cells expressed both flt-1 and flk-1. At day 5, immunoreactivity for both VEGF and its receptors was detected in decidual cells and vascular endothelial cells. Only a few embryonic cells expressed VEGF mRNA but strong signals were noted in decidual cells. The patterns of VEGF and VEGF receptor expression were the same in the day-6 and day-7 embryos and decidua, except for an increase in intensity as development progressed. Based on these findings, we conclude that, in addition to its known actions on endometrial angiogenesis and tissue swelling, VEGF may also facilitate the proliferation and differentiation of the endometrium and help to sustain the avascular embryo during this early stage of development.