en Original Article Original Article <strong>Background:</strong> The risk of multiple pregnancies often present in programs of In Vitro Fertilization (IVF) is an important force for embryo cryopreservation. On the other hand ethical restriction and assurance of potential fertility following chemo/radio therapy has led scientists to focus on female gamete preservation.<br> <strong>Objective:</strong> Optimizing vitrification protocol by using less concentrated cryoprotectants (CPAs) in order to decrease CPAs toxicity.<br> <strong>Materials and Methods:</strong> Mouse Metaphase-II (M-II) oocytes and four cell-stage embryos were collected. Oocytes Survival Fertilization and Developmental Rates (SRs FRs DRs) were recorded after cryotop-vitrification/warming. As well as comparing fresh oocytes and embryos the data obtained from experimental groups (exp.) applying 1.25 1.0 0.75 molar (M) CPAs were analyzed in comparison to those of adopting 1.5 M CPAs [largely-used concentration of Ethylen Glycol (EG) and Dimethyl-sulphoxide (DMSO)].<br> <strong>Results: </strong>The data of oocytes exposed to 1.25 M concentrated CPAs were in consistency with those exposed to 1.5 M and control group in terms of SR FR and DR. As less concentration was applied the more decreased SRs FRs and DRs were obtained from other experimental groups. The results of embryos which were exposed to 1.25 M and 1.0 M were close to those vitrified with 1.5 M and fresh embryos. The results of 0.75 M concentrated CPAs solutions were significantly lower than those of control 1.5 M and 1.0 M treated groups.<br> <strong>Conclusion: </strong>CPAs limited reduction to 1.25 M and 1.0 M instead of using 1.5 M for oocyte and embryo cryotop-vitrification procedure may be a slight adjustment. Cryotop, Cryoprotectant, Embryo, Mouse, Oocyte, Vitrification 181 188 http://ijrm.ir/browse.php?a_code=A-10-549-231&slc_lang=en&sid=1 Sahar Almasi-Turk No Molecular and Cell Biology Research Center, Shaheed Beheshti University of Medical Sciences, Tehran, Iran Amrollah Roozbehi No Department of Anatomical Sciences, Yasuj University of Medical Sciences, Yasuj, Iran Elham Aliabadi No Department of Anatomical Sciences, Shiraz University of Medical Sciences, Shiraz, Iran Ali Haeri No Department of Pharmacology, Shaheed Beheshti University of Medical Sciences, Tehran, Iran Yoosef Sadeghi No Molecular and Cell Biology Research Center, Shaheed Beheshti University of Medical Sciences, Tehran, Iran Ahmad Hosseini prof_hosseini@yahoo.com Yes Molecular and Cell Biology Research Center, Shaheed Beheshti University of Medical Sciences, Tehran, Iran