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The present study was conducted to examine the development of nuclear transplant embryos produced by transplanting nuclei to either oocytes or zygotes in the mouse. Metaphase II oocytes and one-cell zygotes were enucleated and fused with transferred nuclei from late two-, four- and eight-cell stage embryos. Enucleation of metaphase oocytes was achieved using the interference micro-scope without staining. Fusion and oocyte activation were performed by means of electric fields. Similar development rates to the blastocyst stage were obtained from enucleated oocytes (28.0%) and zygotes (30.9%) reconstituted with nuclei from late two-cell embryos. Cleavage and blastocoele formation of reconstituted embryos occurred at around the same time as observed in the control embryos, with some exceptions. After transfer to recipient females, live young were obtained from both reconstituted oocytes (9.1%) and zygotes (11. 5%) that received a nucleus from late two-cell embryos. The results indi-cate that enucleated zygotes as well as oocytes can support development to term of nuclei introduced from late two-cell embryos in which activation of the embryonic genome has occured, which may be a result of the reprogramming of the donor nucleus. Key words: Nuclear transplantation, nuclear reprogramming, mous