The localization of enhanced green fluorescent protein (EGFP) was studied in preimplantation embryos obtained from reciprocal mating of hemizygous C57Bl/6-Tgn (ACTbEGFP)1Osb/J mice with C57Bl/6 mice. Specific fluorescence of EGFP was observed in all oocytes and embryos obtained from transgenic females during all preimplantation stages and in embryos inheriting the EGFP gene from transgenic males starting from the 8 blastomere stage during the compactization period. EGFP mRNA or EGFP synthesized during oogenesis can be retained in embryos during the entire preimplantation period, while expression of EGFP gene transferred from the father coincides with the onset of compactization. The possibility of using these embryos in experimental mammalian embryology is discussed.