These experiments were carried out to examine the effect of rapid thawing (500$^{circ}C$/min) on the survival of 8-cell mouse embryos cooled slowly (0.5-1$^{circ}C$/min) to precooling temperatures between -10 and 070$^{circ}C$ before direct transfer ofembryos to -196$^{circ}C$, and to compare the survival of embroys thawed slowly (20$^{circ}C$/min) and rapidly (500$^{circ}C$/min) after in vitro culture. In addition, the sensitivity of 8-cell mouse embroys to the rate of addition and removal of cryoprotectant at room temperauture, and the effect of developing stages on the survival of embryos frozen-thawed slowly were investigated. The results obtained were summarized as follows: 1. Embryos were survived in rapid thawing (500$^{circ}C$/min) only when slow cooling was terminated at relatively high subzero teperaure (-20 to -60$^{circ}C$). The highest survival rate(77.0%) in in vitro culture of embryos thawed rapidly was obtaeined after transfer to -196$^{circ}C$ from -40$^{circ}C$. 2. For the survival of embryos in slow thawing (20$^{circ}C$/min.), slow cooling to lower subzero temperature (-50$^{circ}C$ and below) was required before transfer of embryos to -196$^{circ}C$. These results indicate that embryos transferred to -196$^{circ}C$ from high subzero temprature contain much interacellular ice to damage them during slow warming but to permit survival of embryos after rapid warming. 3. The Survival rate (72.7%) of 8-cell mouse embryos after rapid addition and removal of cryoprotectant, DMSO at room temperature was similar to that (83.9%) after slow addition and removal of cryoprotectant at same temperature. 4. The survival rates of 1-, 2-, 4- and 8-cell embryos frozen-thawed slowly were 26.7, 76.4, 70.0 and 83.9%, respectively.