An ultrastructural comparison of mouse oocytes isolated at various stages of growth and meiotic competence has been carried out. Progressive changes in the nucleoli, ribosomes, mitochondria, endoplasmic reticulum, Golgi complex, and other organelles and inclusions of the oocyte have been examined as a function of oocyte size by transmission electron microscopy. The observations presented support the idea that growth of the mammalian oocyte involves not just tremendous enlargement of the cell, but extensive alterations in its overall metabolism as reflected in the ultrastructure of the oocyte at various stages of growth.