Ovaries contain a huge number of non-growing and growing oocytes. Once non-growing oocytes (pig and cow: 30 microm in diameter) in primordial follicles enter the growth phase, they grow toward their final size (120-125 microm) taking a long period of time. The small oocytes have no ability to mature because of the inability to activate Cdc2 kinase and MAP kinase that are required for maturation. During the final growth phase, oocytes acquire the ability to activate these kinases. The population of the oocytes that grow to the final size is quite small in the ovary. Artificially growing-up of small oocytes could provide a new source of mature eggs for livestock production and assisted reproduction in humans. Baby mice have been produced by in vitro grown oocytes from primordial follicles. In large domestic species, only two baby calves have been produced from cultured oocytes that were at the mid-growth phase (90-99 microm) from early antral follicles. A culture system for the oocytes in secondary or smaller follicles has not been established. Xenotransplantation of oocytes to immunodeficient mice is a substitute for the culture. Bovine secondary follicles (oocyte: 55 microm) developed to the antral stage with oocytes reaching their final size in xenografts after 2 months. The grown oocytes matured and were penetrated by spermatozoa. Bovine and porcine primordial follicles (oocyte: 30 microm) developed to the antral stage after 6 months. In vitro growth and xenotransplantation systems will provide a new understanding of the mechanisms regulating oogenesis and folliculogenesis in the ovary.