The purpose of this study was to assess the effect of oxygen condition on the survival and growth in vitro of bovine oocyte-cumulus/granulosa cell complexes isolated from early antral follicles. Complexes comprised of a growing oocyte enclosed in cumulus layers and a piece of mural granulosa cells were isolated from early antral follicles, 0.5-0.7 mm in diameter, and cultured for 14 days in TCM199 supplemented with 5% fetal bovine serum and 4 mM hypoxanthine in an atmosphere of 5% O₂ or 20% O₂. Mean oocyte diameter at the beginning of culture was about 95 µm. Within 4 days formation of the antrum-like structure was evident in virtually all complexes,regardless of high or low oxygen tension, but many of the complexes failed to retain this structure after 8 days. At low oxygen tension, complexes retain the antrum-like structure for slightly longer periods than those cultured at high oxygen tension, yet no significant difference was found. Accordingly, the percentage of oocytes that survived in culture was higher at low oxygen tension. Nevertheless, the oocytes (105.9 ± 8.4 µm) were smaller than those grown at high oxygen tension (112.6 ± 10.5 µm) (P<0.05). Supplementation of estradiol-17β was effective in increasing the percentage of follicles that retained the cavity, especially at low oxygen tension, and of oocytes recovered as being enclosed with cumulus cells after 14-day cultures, but no promotion of oocyte growth caused by estradiol-17β was observed. Taken together, a reduced oxygen condition may be beneficial in improving the survival rate of oocytes, but not in promoting growth in the culture system used in this study.