Resumption of the meiotic maturation and extruded the first polar body of the murine oocytes was registered at 2 and 14 hours under culturing in the medium containing 1.7 mM Ca2+. There were differences between the groups in the percentage of GV oocytes with zona pellucida that acquired competence to undergo germinal vesicle breakdown (GVBD) at 2 hr time point 22.2% of "small" and 32.4% of "large" follicles of mouse at dioestrus and, respectively, 28.1% and 41.8% follicles of mouse at oestrous underwent GVBD. At 14 time point 21.4% of "small" and 30.3% of "large" follicles of mice at mice at dioestrus and, respectively, 26.1% and 43.6% follicles of mouse at oestrous underwent GVBD and (then) extruded the first polar body (FPB). It may be suggested, that acquisition of competence of the oocytes to resume meiosis in vitro depends on the size of follicles and the stage of murine oestrous cycle. Thus, the largest percentage of GV oocytes resuming meiosis was from "largest" follicles of mouse at oestrous.