Mouse oocytes obtained at various times after administration of HCG or after culture were studied with the electron microscope. Gradual changes were observed in the perivitelline space, plasma membrane and cortical cytoplasm. Disappearance of the regular microvilli, the formation of lamellae-like processes and vesicular bodies were observed. A progressive increase in the number of cortical granules and multivesicular complexes was observed simultaneously with the disappearance of the rough endoplasmic reticulum. In general, eggs which were matured either in vivo or in vitro showed similar patterns of morphological differentiation. Degenerative changes were seen in most eggs cultured for 17 hours, but not for nine hours or less. In aged tubal eggs recovered at 26 and 36 hours after injection of HCG, the number of cortical granules was decreased relative to previous stages and their extrusion from the egg was apparent. The fertilizability of the eggs matured in vitro and the role of the cortical granules are discussed in terms of the reaction of the zona pellucida.