Eight-cell mouse embryos were cryopreserved by vitrification in a concentrated solution of dimethylsulphoxide, acetamide, propylene glycol and polyethylene glycol. This solution (designated VS1) does not crystallize when cooled to subzero temperatures but instead forms a glassy transparent solid. Embryos were exposed in three steps to a stock VS1 solution or a saline solution containing 90% of the cryoprotectants in the stock VS1 (90% VS1) and then the suspensions were vitrified by rapid cooling in liquid nitrogen. Of 568 embryos vitrified in 90% VS1, 80% developed in vitro and 98 normal fetuses or young (17% of the total) were produced after transfer to pseudopregnant recipients. By contrast, 22% of 153 embryos vitrified in the stock VS1 developed in vitro, but only one normal fetus was obtained after transfer. These results demonstrate that normal fetuses and young can be produced from embryos cryopreserved by the simple and rapid method of vitrification.