The mRNA expression patterns of EGF, HB-EGF, Amphiregulin, EGF receptor, IGF-1, CSF-1, IL-1 alpha, IL-1 beta, IL-1 receptor type 1, IL-1 receptor antagonist, LIF, COX-1, COX-2, Mucin-1, calcitonin, and rat USAG-1 mouse homologue, all of which are involved in the process of conceptus implantation to the endometrium, were examined during the estrous cycle by means of real-time quantitative PCR. COX-2, HB-EGF, LIF, Mucin-1, CSF-1, IL-1 alpha, IL-1 beta, and IL-1 receptor antagonist were temporally regulated during the estrous cycle and highly expressed during the estrous stage. In the case of COX-1, EGF, IGF-1, and EGF receptor, the highest mRNA expression was during the diestrous stage. In contrast, the rat USAG-1 mouse homologue mRNA expression did not change during the estrous cycle. These results indicate that rat USAG-1 mouse homologue expression at implantation might be specifically regulated by embryonic factors rather than the maternal environment.