Abstract: In the present study, the DNA methylation patterns of in vitro-derived mouse tetraploid embryos were investi-gated by immunofluorescence staining with an antibody against 5-methylcytosine (5MeC). Tetraploid embryos could be produced by electrofusion at the stage of two-cell embryos, which could develop to blastocysts followed by fusion of cyto-plasm and nucleus and cleavage in vitro. During the fusion of cytoplasm, the DNA methylation levels of the fused embryos are as high as these of two-cell diploid embryos in vivo Then the embryos are rapidly demethylated when the nucleus begin to fuse, resulting in the lowest DNA methylation levels when the nucleus are fused completely. After that, the DNA methy-lation levels of the fused embryos are gradually increased until the morula stage. However, whereas an asymmetric distribu-tion of DNA methylation is established in vivo-derived blastocysts with a higher methylation level in the inner cell mass (ICM) than that in the trophectoderm, we can not detect the asymmetric distribution in most in vitro-derived tetraploid blastocysts. So the DNA methylation patterns of mouse tetraploid embryos are aberrant, which may lead to subsequent de-velopmental failure and embryo death. This is the first report on the methylation patterns of in vitro-derived mouse tetraploid embryos.