Cell volume homeostasis is particularly critical for early preimplantation development [1]. The ability to regulate cell size first appears in the oocyte after ovulation is triggered [2]. The nature of the regulatory mechanisms governing the volume changes in the mature oocyte during regulatory volume decrease (RVD) is not clear. Volume-sensitive organic osmolyte-anion channel or VSOAC is known to be involved in RVD in most somatic cells. It was shown that VSOAC-like channel may also participate in egg volume regulation [3]. The most important property of VSOAC is the open-channel block by millimolar concentrations of extracellular ATP [4]. The aim of this study was to investigate the external ATP influence on kinetics of mouse metaphase II (MII) oocyte osmotic response under hypotonic stress. Eggs (MII oocytes) were obtained from 8-10-week-old SHK female mice. The females were superovulated by an i.p. injection of 10 IU of Folligon (Intervet, Holland ) followed by an i.p. injection of 5 IU of human chorionic gonadotropin (hCG, Intervet, Holland) 46-48 hs later. The MII oocytes were flushed from excised oviducts of sacrificed by cervical dislocation animals 14 hs after hCG administration. The hypotonicity was created by replacing 140mM NaCl in Dulbecco`s solution with 70mM NaCl. Eggs were exposed for 90 minutes in the hypotonic conditions. The similar procedure was employed for Dulbecco`s containing 5 mM ATP (Sigma, USA). Osmotic adaptation in single oocyte has been studied employing the direct measurement of cell volume with the quantitative microtomography [5]. Egg volume was determined with laser scanning microscopy followed by three-dimensional reconstruction (3-DR). The keeping of the intact volume of mature mouse oocyte was based on freeze-drying technique. After cryofixation in liquid propane and subsequent low-temperature dehydration the cell was immediately immersed in the Epon medium. The monolayer of freeze-dried embedded oocytes was examined with a laser scanning microscope (Leica DM2500, Germany). Using 532-nm laser a gallery of optical slices of mouse oocyte was collected in Z-direction with deepness step of 1 µm thick. 3-DR was performed in the 3ds max medium. All results are presented as means ± S.D., compared by Student’s t-test. Our data indicate that hypoosmotic incubation for 20 minutes resulted in the swelling peak of oocyte volume of 221±15 pl vs. initial level of 121±13 pl, p<0.05. After 90 minute exposure the cell volume was recovered to 174±24 pl vs. 213±17 pl in hypotonic Dulbecco`s with ATP, p<0.05. In the given research it was shown that mouse MII oocytes are capable of RVD. The recovery is pronounced but not completed at 90 minutes. RVD in mouse eggs is inhibited by external ATP of 5mM. This finding suggests the involvement of the VSOAC-like mechanism in osmotic response of mature oocyte to hypotonic stress.