As an initial step to clarify the mechanisms of various uterine actions of PGE₂, expression patterns of the messenger RNAs (mRNAs) for four subtypes of PGE receptors, EP₁, EP₂, EP₃, and EP₄, were investigated in the mouse uterus during pseudopregnancy. Relative expression levels were investigated by Northern blot analysis of mRNA levels in uteri obtained on days 0, 1, 3, 5, 7, and 9 of pseudopregnancy (day 0 = 48 h after PMSG injection), and cellular localization was determined by in situ hybridization in uteri obtained on days 0 and 5. EP₂ mRNA was specifically expressed on day 5, and its expression was confined to the luminal epithelium. On the other hand, the level of the EP₃ mRNA expression progressively increased until day 5. Cell populations expressing the EP₃ mRNA were confined to the longitudinal smooth muscle on day 0, but they changed to the circular smooth muscle on day 5. The expression level of EP₄ mRNA was low on days 0 and 1, but it became high on days 3 and 5. On day 0, EP₄ mRNA was localized to the luminal epithelium. On day 5, diffuse, but significant, EP₄ expression was observed over the endometrial stroma and epithelium. No EP₁ mRNA signals were observed. Transient expression of EP₂ on day 5 of pseudopregnancy in the luminal epithelium suggests its involvement in blastocyst implantation signaling. EP₄ in the endometrial stroma is suggested to be involved in decidual transformation of the stromal cells, whereas EP₃ in the myometrium is believed to be involved in regulation of myometrial activity.