DNA methylation is crucial for mammalian development, and DNA methylation is always in the dynamic status during preimplantation mouse embryos development. The effects of 5-AZA-CdR on the development of preimplantation mouse embryos were evaluated. Preimplantation mouse embryos created by in vitro fertilization were cultured continuously in 5-AZA-CdR (0.2, 1.0, or 5.0 μmol/L). Fertilized oocytes exposed to CZB containing 5-AZA-CdR at the pronuclear stage were unable to form morulae (0.2 and 1.0 μmol/L) or 4-cell embryos (5.0 μmol/L), while 2-cell stage embryos exposed to 5-AZA-CdR developed into uncompacted 8-cell (0.2 and 1.0 μmol/L) or 3/4-cell (5.0 μmol/L) stage embryos. The rate of morula formation was significantly lower in 4-cell embryos cultured in 5-AZA-CdR (1.0 or 5.0 μmol/L) than that in control embryos ( P < 0.05). These data indicate that 5-AZA-CdR inhibits the development of mouse preimplantation embryos. Apoptosis, DNA methylation, and transcriptional activity were analyzed to determine the reason for these developmental defects. An annexin V-PI assay revealed that high doses of 5-AZA-CdR led to apoptosis. Compared to the controls, DNA methylation was significantly reduced in uncompacted 8-cell embryos and morulae ( P < 0.05) in a dose- dependent manner, whereas no significant change was detected in 2- or 4-cell embryos ( P > 0.05). The observed changes in transcriptional activity, determined by measuring the incorporation of BrUTP, were similar to the observed alterations in DNA methylation. Therefore, the developmental defects induced by 5-AZA-CdR appear to be mediated by alterations in DNA methylation and transcriptional activity in preimplantation mouse embryos.