Half grown oocytes were released from follicles of 7-day-old mouse ovaries during overnight culture. Naked oocytes free of attached follicular cells were cultured in MEM plus 10% fetal calf serum over the developing monolayer of ovarian cells. Oocytes increased in diameter for a period of at least a week at 1.6 micron per day, or 70% of the in vivo growth rate. The ultrastructure of cytoplasmic organelles in cultured growing oocytes was normal. During the second week, about 35% underwent spontaneous fragmentation, and at least 10-20% resumed meiotic maturation. We conclude that a significant fraction of naked oocytes reached a functional state comparable to that of normal full grown oocytes at a rate approaching the in vivo rate. Free oocytes with attached cells grew to a larger average diameter than naked oocytes, and incorporated 3H-leucine 75% more rapidly. Naked oocytes cultured in the absence of ovarian cells did not grow, and died when pyruvate was omitted from the medium. Naked oocytes did not grow when cultured over primary mouse fibroblasts, L-cells, CHO cells, or hepatoma cells. Naked oocytes separated from the monolayer of ovarian cells by a 0.7 mm layer of agar grew at 60% of the rate of control oocytes.