Although estrogen is recognized as essential for embryonic development and maintenance of pregnancy, it remains unclear whether it has a direct role in the embryos themselves. The aim of this study was to investigate whether estrogen can have any effect in pig embryos during preimplantation development. Since the function of estrogen is mediated through its specific receptor, estrogen receptor (ER), the presence of ER mRNA and protein in pig embryos collected in vivo at different stages of preimplantation development was determined and compared. Using reverse transcription polymerase chain reaction, ER RNA was detected at the one-cell, two-cell, and four-cell stages. The level became undetectable at the five-to eight-cell stages and the morula stages and then reappeared again at the blastocyst stage. To determine whether the ER message observed in the embryos was translated into ER protein, immunocytochemical analysis was performed and the presence of ER protein was detected in oocytes at one-cell and four-cell stages. However, the amount of ER protein in porcine embryos at the blastocyst stage was still below the detection limit. The presence of ER mRNA at the blastocyst stages suggests that estrogen may start to act directly on pig embryos afterwards, and our results provide a basis for determining the direct role of estrogen in preimplantation pig embryos.