Objective: The animal cells can readjust their volume after transient osmotic swelling by a mechanism known as regulatory volume decrease (RVD). The nature of the regulatory mechanisms governing the volume changes in embryonic cell during RVD is not clear. The purpose of the study was to determine the contribution of Na⁺ /K⁺-ATPase, pH and cytoskeleton to regulatory volume decrease of mouse blastomere. Methods: The keeping of the intact volume of two-cell mouse embryo was based on freeze-drying technique. The hypotonicity was created by replacing 140mM NaCl in Dulbecco`s solution with 70mM NaCl. Osmotic adaptation in a blastomere has been studied employing the direct measurement of cell volume with laser scanning microscopy followed by three- dimensional reconstruction. Results: Our data indicate that hypoosmotic incubation for 10 minutes resulted in the swelling peak of blastomere volume (94±8) x 10<sup>3</sup>mm<sup>3</sup> (these and all results below are presented as the mean and the standard deviation). After 60 minute incubation the cell volume was recovered to control level (57±4)x 10<sup>3</sup>mm<sup>3</sup>. Exposure of early mouse embryo to the hypotonic K<sup>+</sup> -free Dulbecco`s did not influence on the kinetics of cell osmotic response. During incubation in the same medium at pH=6,3 blastomere exhibited similar swelling and shrinkage behavior with the swelling peak (77±10) x 10³mm³. After 60 minute incubation in the hypotonic Dulbecco`s with Cytochalasin B blastomere's volume was calculated as (86±6) x 10³mm³. Conclusion: Our results indicate that Na⁺ /K⁺ -ATPase inhibition and pH decrease do not influence on volume recovery of the embryonic cells, but blastomere shrinkage is abolished by Cytochalasine B treatment. We conclude that RVD specific for hypotonic shock is blocked by the alteration of F-actin organization.