The molecular organization of the oocyte plasma membrane required for sperm docking and the corresponding elements that determine where fertilization can occur have been identified. We found that their organization is directly linked to the state of mitochondrial polarization in the subplasmalemmal cytoplasm, and persistent hypo-polarization is directly associated with fertilization failure in women. However, regional hypo-polarization is a normal feature of the perinuclear oolemma in the mature mouse oocyte where sperm cannot attach. At present, we are investigating why these mitochondria normally remain hypo-polarized and what effects this may have on certain elements of the oolemma found to be essential for sperm docking and penetration.