Survival of 8-cell mouse embryos of inbred strains C57B1 and DBA and outbred strain NMRI was studied after ultrafast freezing. The embryos were placed in 10% glycerol for 10 min, transferred in plastic tubes filled with a mixture of glycerol and 1 M sucrose (3:7), and immersed in liquid nitrogen within 1.5 min. The tubes were thawed in air at the room temperature, the embryos were washed in 0.5 M sucrose and cultivated in a Dulbecco medium with 20% fetal calf serum at 37 degrees C for 48 h or transplanted to recipient females. The survival of embryos was estimated according to their capacity to develop in vitro to the blastocyst stage or form normal fetuses after transplantation. After thawing and cultivation, 80.4% of NMRI embryos retained viability (80.7% in the control). For C57B1 and DBA embryos, these indices were 90.2 and 88.7%, and 59.3 and 66.7%, respectively. The differences between the experiment and control was insignificant for all strains. After thawing and transplantation of NMRI embryos, 34.4% developed into normal live fetuses (in the control 33.3%). At the same time, after cryoconservation by the standard method using a programmed freezer, the viability of ICR and NMRI embryos decreased approximately by 20% even in in vitro experiments. Thus, no differences were found in survival of the embryos of the studied strains after ultrafast freezing. This method proved to be very efficient for the embryos of both inbred and outbred mouse strains, since their capacity for development practically did not decrease after cryoconservation. The method may be used for production of cryobanks of embryos of various mouse strains.