Early preimplantation embryos are sensitive to external osmolarity and use novel mechanisms to accumulate organic osmolytes and thus control their cell volumes and maintain viability. However, these mechanisms are restricted to the cleavage stages of development, and it was unknown whether postcompaction embryos use organic osmolytes. Mouse embryos developing from the 8-cell stage formed blastocoel cavities in vitro at osmolarities up to 360 mOsM. Above this range, several putative organic osmolytes (alanine, glutamine, glycine, and beta-alanine) rescued blastocyst development, but several effective osmoprotectants in cleavage-stage embryos (such as betaine and proline) did not. At physiological osmolarities, each of these compounds resulted in significantly larger blastocysts. This was not due to increased cell numbers, which were unaffected in blastocysts by osmolarity in the range where blastocyst size was rescued by potential organic osmolytes, although cell number was decreased at higher osmolarities and was rescued by each osmolyte. The effective osmolytes were accumulated intracellularly by embryos developing in vitro from the 8-cell stage to blastocysts. However, unlike conventional organic osmolytes in somatic cells or those in cleavage-stage embryos, their intracellular concentrations were not increased with increasing external osmolarity. With the exception of beta-alanine, which is taken up via the beta-amino acid transport system, the effective osmolytes were transported by the B^0, system, which becomes highly active in blastocysts. The intracellular accumulation of these osmolytes in postcompaction embryos thus appears to support optimal development and blastocyst expansion at physiological osmolarities and may contribute to the embryo's ability to withstand stress.