Oocytes recovered from or retained within the follicles dissected from ovaries of juvenile marmosets were cultured to examine the effect of follicular size, i.e. large (>800μm), medium (500-800μm) and small (<500μm), on the resumption and completion of meiotic maturation.The follicle-enclosed oocytes from all follicular groups failed to resume meiosis beyond the germinal vesicle stage, whereas the oocyte-cumulus cell complexes underwent the first meiotic division at a frequency which increased with the follicular growth. Maturation rates were 42.9% of oocytes from large, 22.2% from medium and 11.5% from small follicles. The incidence of degenerated oocytes decreased significantly as the fol-licular size increased.It is concluded that isolated oocytes from ovarian follicles of juvenile marmosets resume meiosis spontaneously in vitro and it was suggested that the ability to complete meiotic maturation is acquired during the stage of follicular growth.