Abstract:Objective: Two-cell block as a problem occurs in some couples referring to infertility center. This study was designed to compare the effect of different kinds of chemical activators on arrested mouse two-cell stage embryos in order to enhance cleavage and developmental formation rate. Material and Methods: Following superovulation, the female mice were mated with males and positive vaginal plaque mice were euthanized 48 hours after hCG injection. Subsequently, 2-cell embryos were collected and randomly cultured (in M16 medium) in six groups. Some embryos were washed and cultured as 1st group without any exposure. The remaining 2-cell stage embryos were exposed to 4°C for 24 hours in order to arrest in 2-cell stage for 2nd to 6th groups. The 2nd group was incubated immediately, while the 3rd group was exposed to 10 µM Ionomycine for 3 minutes and the 4th group was exposed to 10 mM strontium for 5 minutes. The 5th group was exposed to %0.1 Ethanol for 5 minutes and the 6th group to %0.1 Methanol for 3 minutes. Subsequently, all groups were incubated up to blastocyst stage. Results: Data were analysed employing a one-way Anova test the results show that the rate of degenerated embryos is significantly different (P<0.05) between groups by low temperature (4°C) exposure. The mean percentages of cleavage, blastocyst and hatched blastocyst formation rate in the 4th group were 80.9%, 69.2%, and 46% respectively, showing a significant difference between groups. Conclusion: This study shows that among different chemical activators used in this study, Strontium is the most powerful chemical activator to enhance cleavage and development of arrested two-cell embryos in the 4 th group.