Structural changes in cumulus-oocyte complexes and granulosa cells were induced by administering pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG) to female mice. Three and 9h after hCG administration, dissolution of the germinal vesicle (GVBD) and progression to the second metaphase occurred respectively in oocytes. The number of cumulus cells associated with the granulosa cell layer decreased significantly 1h after hCG administration. Expansion of cumulus cell investment was due to the abundant deposition of intercellular materials in the cumulus-oocyte complexes during oocyte maturation. A strongly negative-charged (colloidal iron-positive) substance was detected in the intercellular spaces of follicular tissue, especially in the cumulus mass. Moreover, cells located where the cumulus mass and granulosa cell layer intertwined became enlarged during the resumption of the meiosis of oocytes. Colloidal iron-positive substances accumulated extensively within the intercellular spaces of the enlarged cells.