Osmolarity of Dulbecco's medium at which the volume of two-cell mouse embryo remained similar to that of intact embryo was determined. The method is based on comparison of kinetic curves describing the volume of embryonic cell in solutions of different osmolarity. The blastomere volume was measured by quantitative laser microtomography after fixed osmotic stress intervals. It was found that Dulbecco's saline with 125 mM NaCl solution is an isotonic solution for two-cell mouse embryo. This concentration corresponds to 290 mOsm, which is lower than osmolarity (~310 mOsm) of media routinely used for culturing of differentiated cells or biological fluids, e.g. blood plasma.