Uterine receptivity is prerequisite for the attachment of the embryo to the uterine epithelium and involves a specialized polarity-dependent property of uterine epithelial (UE) cells. These UE cells, when polarized in culture, behave like cells in utero by exhibiting apico-basal polarity. In order to develop an implantation model in vitro, UE cells were polarized on extracellular matrix (ECM), and polarity was validated by response to estradiol-17beta administered exogenously. UE cells of pregnant rats at day-3 and day-4 post-coitum (p.c.) and of non-pregnant rats were cultured on bare and extracellular-matrix-coated petri dishes until confluency. Hatched blastocysts were transferred to the cultures, and adhesion was monitored every 24 h. Although blastocysts attached to UE cells that were taken from non-pregnant rats and from rats of day-3 p.c. and cultured on bare plastic, they failed to attach to these cells polarized on ECM. However, blastocysts attached firmly to UE cells that had been taken from rats of day-4 p.c. and polarized on ECM. Receptivity of UE cells taken from non-pregnant and pregnant (day-4 p.c.) rats was quantitated by flow cytometric estimation of cellular levels of beta3 integrin. The expression of beta3 integrin in UE cells from rats of day-4 p.c. was highly significant (P<0.01) when compared with its expression in UE cells from non-pregnant rats. The expression of beta3 integrin in UE cells of day-4 p.c. confirmed the receptivity of these cells to blastocyst implantation. Uterine receptivity was also validated in vivo by inducing the decidual cell reaction in rats ovariectomized on day-3 and day-4 p.c. Whereas remarkable deciduoma was noticed in the animals of day-4 p.c., it was absent in the animals of day-3 p.c., thereby indicating that the uterus was receptive on day-4 p.c. only. Thus, blastocysts do not attach to polarized UE cells that have been obtained from a non-receptive uterus. Attachment will occur only if the cells are obtained from a receptive uterus. UE cell receptivity is therefore essential for mimicking the process of implantation in vitro.