At fertilization, repetitive transient rises of intracellular calcium concentration occur in all mammals studied so far. It has been shown that calcium rises could be induced when mouse fertilized 1-, 2-cell nuclei were transplanted into unfertilized eggs and that the reconstituted embryo could be activated. However, whether the capability of inducing calcium rises occurs in all stages of mammalian embryos remains unknown. In this study, by using the nuclear transplantation technique and measurement of intracellular calcium rises in living cells, we showed that only the nuclei from mouse fertilized 1-cell and 2-cell embryos, neither the nuclei from 4-, 8-cell and ethanol activated parthenogenetic embryos nor 2 or 3 nuclei of electrofused 4-cell stage syncytium, have calcium-releasing activity when they were transferred into unfertilized mature oocytes. Our results indicate that the calcium-releasing activity in nuclei of 1-, 2-cell embryos is produced during fertilization and exists at the special stage of fertilized early embryos. These suggested that the capacity of inducing calcium release activity in fertilized early embryos is important for normal embryonic development.