Four-cell mouse embryos obtained by fertilization in vitro were frozen ultrarapidly, immediately after one-step exposure to a highly concentrated solution (modified VS1) at room temperature, and later thawed in a 37 degrees C waterbath. The proportion of morphologically normal embryos on thawing was 95.7% (198/207), and 83.8% (166/198) of these embryos developed to morulas and early blastocysts after 24 hr in culture. All frozen-thawed embryos that developed to morulas and early blastocysts were transferred to the uterine horns of recipients on day 3 of pseudopregnancy and 47.0% (78/166) of transferred embryos developed to normal young.