en Original Article Original Article <strong>Background:</strong> In vitro maturation (IVM) of oocytes reduces the costs and averts the side-effects of gonadotropin stimulation for in vitro fertilization (IVF). Reliable IVM is an intellectual, scientific and clinical challenge with a number of potential applications.<br> <strong>Objective:</strong> The effect of hCG was evaluated on the timing and regulation of in vitro ovulation for the Syrian mice oocytes in the presence and absence of FSH.&nbsp;<br> <strong>Materials and Methods</strong>: Preantral follicles, isolated from the ovaries of 6 weeks-old mice, were cultured in TCM-199 medium. The effect of 10-200 mIU/ml FSH and 1.5 IU/ml hCG was seen on the follicle maturation, as well as the changes in ovulation capacity of enclosed oocytes, after the incubation period of 6 days at 37 &deg;C, 92% humidity and 5% CO₂ in air.<br> <strong>Results:</strong> 100 mIU/ml FSH showed increased follicle diameter, survival, germinal vesicle breakdown (GVBD) and oocyte maturation rates (p<0.0001). Significantly higher number of follicles showed cumulus attachment when ovulation started within 16-24 hours post hCG (97% and 80% respectively; p<0.0001) as compared to the cultures without hCG or when the ovulation time increased from 24 hours post hCG. Combination of FSH and hCG showed 97% (p<0.0001) ovulation as compared to that seen for FSH-containing medium only (81%) or control (10%).<br> <strong>Conclusion:</strong> The combined administration of 1.5 IU/ml hCG and 100 mIU/ml FSH induces the in vitro follicle maturation, ovulation capacity and proper timing of mice oocytes.&nbsp; Follicle stimulating hormone, HCG, Preantral follicles, Oocyte maturation, GVBD. 199 204 http://ijrm.ir/browse.php?a_code=A-10-549-264&slc_lang=en&sid=1 Aisha Javed No Research and Clinical Center for Infertility, Shahid Sadoughi University of Medical Sciences, Yazd, Iran Saeed Rezaei-Zarchi No Department of Biology, Payam-e-Noor University, Yazd, Iran Morteza Anvari No Research and Clinical Center for Infertility, Shahid Sadoughi University of Medical Sciences, Yazd, Iran Madiha Javeed Ghani No Department of Bioinformatics, Government College University, Faisalabad, Pakistan Fatemeh Barzegari Firouzabadi No Department of Biology, Payam-e-Noor University, Yazd, Iran Amer Jamil No Department of Biochemistry, University of Agriculture, Faisalabad, Pakistan Seyed Mehdi Kalantar smkalantar@yahoo.com Yes Research and Clinical Center for Infertility, Shahid Sadoughi University of Medical Sciences, Yazd, Iran Habibollah Nazem No Department of Biology, Payam-e-Noor University, Yazd, Iran