en Original Article Original Article <strong>Background:</strong> Preparation of oocytes is one of the critical factors that determine the developmental competence of embryos produced by in vitro fertilization (IVF).<br> <strong>Objective:</strong> In this study, the effect of cysteamine, type of media and glutathione (GSH) level on blastocysts development after in vitro maturation of mouse oocytes were investigated.<br> <strong>Materials and Methods:</strong> Premature female mice were primed with pregnant mare stimulating gonadotrophin (PMSG), and germinal vesicle (GV) stage oocytes were obtained 45 hr later. GV oocytes were cultured in presence of 0, 50, 100, 200 and 500 &micro;m cysteamine in TCM199 and MEME media. After IVM, MII oocytes were in vitro fertilized (IVF) and in vitro cultured (IVC) in order to observe embryo development. A group of In Vivo Ovulated (IVO) oocytes after priming with PMSG and HCG also were included in this study. 5,5-Dithio-bis (2nitrobenzoic acid) DTNB-recycling protocol was used for GSH assay.<br> <strong>Results: </strong>Rate of IVM and IVF were improved in all oocytes treated with cysteamine in the two medium except 500 &micro;m (81% MII rate in TCM and 64% MII in MEME). Rate of blastocyst in 100 &micro;m cysteamine in TCM1199 and 200 &micro;m in MEME was higher compared to control groups (In TCM 45% and in MEME 35%). In vivo MII and GV oocytes represented the highest and lowest GSH level respectively.<br> <strong>Conclusion:</strong> Our results revealed that the media and concentration of cysteamine can affects on IVM, IVF and rate of blastocysts development on dose dependant manner. Cysteamine, IVM, Blastocysts, GSH, Oocyte, Mouse. 159 163 http://ijrm.ir/browse.php?a_code=A-10-549-304&slc_lang=en&sid=1 Amaneh Mohammadi Roushandeh dinachal@yahoo.com Yes Department of Anatomy, Medicine faculty, Tabriz University of Medical Sciences, Tabriz, Iran Parichehr Pasbakhsh No Department of Anatomy, Medicine faculty, Tehran University of Medical Sciences, Tehran, Iran Zohreh Alizadeh No Department of Anatomy, Medicine faculty, Hamedan University of Medical Sciences, Hamedan, Iran Mehryar Habibi Roudkenar No Research center, Iranian Blood Transfusion Organization, Tehran, Iran