Objective To study the effects of hyperglycemia on preovulatory oocyte maturation, development and granulosa cell apoptosis. Methods Female mice (20-day-old) were used to generate diabetic models. Diabetic group and agematched controls received superovulation with 10 IU of PMSG and hCG. At the appropriate time points following hCG injection (t=0h, 2h, 6h and 10h), the ovaries were removed and then paraffin embedded. The expression of connexin-43 was detected by immunohistochemistry. The apoptosis of granulosa cells observed was by TUNEL staining. Results 1.Both the ovaries’ wet weight ratio to the body weight and the number of eggs ovulated were lower in the hyperglycemia group than those in the control group. 2.The expression of connexin-43 detected by immunohistochemistry in the follicles in the hyperglycemia group was greater than that in the control group at the 6th and 10th hours after hCG injection. 3.There were remarkably more TUNELpositive follicles in the diabetic follicles than in the nondiabetic group. And the difference was the most notable at the 6th and 10th hours after hCG injection. 4.Lower percentage of oocytes reached GVBD in the diabetic than that in the controls at the 6th and 10th hours.Conclusion Delayed maturation was observed in the oocytes from those hyperglycemia mice. Hyperglycemia increased granulosa cell apoptosis. The early developmental delay of oocytes may correlate to the