OBJECTIVE To analyze the effect of various culture periods and FSH on oocyte competence in cultured mouse follicles. METHODS Controlled in vitro laboratory experiment. METHODS Academic research environment. METHODS DNA staining, maturation induction, parthenogenesis, and IVF were performed after various periods of oocyte culture. Additionally, FSH modulation during preantral follicle growth was studied. METHODS Nucleolar conformation, meiotic resumption, activation potential, and preimplantation development were recorded. RESULTS Surrounded nucleolus (SN) conformation was found in oocytes cultured for 8 days and correlated by time with meiotic competence. Culture for 12 days increased oocyte diameter (day 8 69.8 mum vs. day 12 72.4 mum) and meiotic competence (day 8 63% vs. day 12 81% polar body oocytes), but decreased fertilization rate (day 8 75% vs. day 12 53% 2-cell embryos). Equal activation potential (49%) on both days indicated that impaired sperm entry could not account for lower fertilization rate after 12 days. Depriving early follicular growth of FSH delayed antrum formation but not SN conformation. CONCLUSIONS Fertilization of cultured oocytes is most efficient close to the completion of SN. Omitting FSH from early follicle growth could not postpone the early onset of SN, which seems to be imposed by the time spent in vitro.