To survive cryopreservation, oocytes, zygotes and embryos must tolerate a sequence of volumetric contractions and expansions. These result as an egg or an embryo is exposed to a permeating cryoprotective additive, then to an increase followed by a decrease in the osmolality of its extracellular milieu as water freezes during cooling and then melts during warming, and finally to the dilution of the cryoprotective additive solution. Measurements of the extent to which mouse zygotes and human oocytes undergo osmotic contraction have been made by exposing them to solutions of monosaccharides (fructose, galactose, glucose) or disaccharides (maltose, sucrose, trehalose), ranging in concentration from 0.25 to 1.50 M. Mouse zygotes and human oocytes exhibit very similar responses to these solutions. Their volumes contract linearly as a function of 1/(osmolality) of the solutions, yielding estimates of non-osmotic volumes of 13-23%. Mouse zygotes exposed to 1.5 M concentrations of these solutions for 10 min lose 85% of their cell water. Yet > 75% of treated zygotes develop into hatching blastocysts. Human oocytes also appear to survive such extreme dehydration, based on a vital dye assay. These results suggest that solutions of various non-permeating saccharides can serve as osmotic buffers for the recovery of cryopreserved oocytes, zygotes and embryos.