This study is designed to detect the emperipolesis in megakaryocyte in fetal liver, which is an important organ of hemopoiesis, during rat development, and to compare the activity of erythropoiesis in fetal liver with that of emperipolesis in megakaryocyte. In order to find that which causes are more related to emperipolesis, we applied periodic acid Schiff reagent, which is special staining method for megakaryocyte and used electron microscope. The size and maturity of magakaryocyte gradually developed with age. The number of megakaryocyte increased in similar proportion to the activity of erythropoiesis. Emperipolesis occurred in more mature megakaryocyte (most stage III). The majority of cells enclosed within megakaryocyte, were the precursor of erythrocytes. Emperipolesis was observed for the first time at 14 day of gestation. The highest frequency of emperipolesis showed 20% of whole megakaryocyte at 16 day of gestation, when the activity of erythropoiesis was most vigorous. The frequency of emperipolesis began to decrease after then, but megakaryocyte was most numerous at 17 day of gestation during fetal and neonatal period. At 19 day of gestation, stage IV megakaryocytes, just before the stage producing platelet, began to appear. Megakaryocyte was not observed after postnatal 10 day. In conclusion, it was found that the emperipolesis in megakaryocyte occurred in the rat fetal liver and was extreme the emperipolesis most observed at the time of most vigorous erythropoiesis during the development of rat fetal liver. It is suggested that the frequency of emperipolesis within megakaryocyte is more closely related with the activity of erythropoiesis in fetal liver than the that of megakaryocytopoiesis, before bone marrow acts as an important organ of hematopoiesis. It is also suggested that the emperipolesis contributes to the production of platelet during gestation period and the maturation of erythrocyte.