The present study was designed to investigate the physiological significance of epidermal growth factor (EGF) receptors expressed on rat uterine epithelial cells during embryo implantation. The number of uterine EGF receptor sites, as determined with ¹²⁵l-labelled EGF as a ligand, progressively increased from the 3rd to the 7th day of pregnancy. Analysis of the EGF receptor by Scatchard transformation of EGF-binding data and affinity labelling with ¹²⁵l-labelled EGF indicated that EGF receptor proteins were noticeably expressed during postimplantation. Determination for EGF receptor sites was further carried out in the uterine tissues of rats, which were superovulating after transplantation of pituitary tissue into a capsule of kidney or subcutaneous injection of 50 IU pregnant mare serum gonadotropin (PMSG) and 50 IU human chorionic gonadotropin (hCG). The transplantation caused an increase in the number of implantation sites (24.5 ± 3.9, n=4) as compared to control rats (14.0 ± 0.6, n=7). Nevertheless, PMSG- and hCG-primed rats totally failed to initiate the implantation process, whereas the number of EGF receptor sites significantly increased during preimplantation (3rd day). In pituitary-transplanted rats on the 7th day, the number of EGF receptor sites was increased as observed in control rats (30.0 ± 0.5 fmol/mg proteins). In PMSG- and hCG-primed rats, however, the EGF receptor was noticeably reduced (7.3 ± 0.4 fmol/mg proteins). These results strongly suggest that functional EGF receptor proteins were noticeably expressed during postimplantation and that its drastic enhancement depends upon the interaction of blastocysts with uterine epithelial cells.