To test in vitro and in vivo the effects of the length of equilibration time and precooling of the vitrification solution, 715 mouse morulae, from strain CF1 Swiss Albino, were cryopreserved by vitrification in 2 Experiments. In Experiment I (n=417) morulae were vitrified in 4 treatments, using 2 equilibration times (5 and 10 min) in the intracellular vitrification solution (VSa) composed of 10% glycerol + 20% 1,2 propanediol and 2 temperatures (4 and 20° C) in the extracellular vitrification solution (VSb) composed of 25% glycerol + 25% 1,2 propanediol. The statistical analysis did not show significative differences among the 4 treatments after thawing and culture in vitro whitin 15 minutes (TI =97%; TII =92%; TIII=92,8%; TIV=94%), 24 hours (TI =90,2%; TII =84%; TIII=83,5%; TIV=89,2%) and 48 hours (TI =79,4%; TII =75%; TIII=73,2%; TIV=77,4%). In Experiment II, 298 Mus musculus morulae were vitrified in 2 treatments. In TI embryos were equilibrated for 5 min in VSa and placed in straws containing the VSb at 20°C. In TII embryos were equilibrated for 10 min in VSa and placed in straws containing the VSb at 4°C. Considering only the pregnant recipients, the implantation rate was 54,2%(TI), 52,7%(TII) and 56,1% (controls), not differing among treatments, although the fetal development rate obtained with TII (45,9%) was significantly higher than TI (30,5%). Data indicate that the lenght of equilibration time and precooling of vitrification solution affected the postthaw in vivo survival of vitrified Mus musculus morulae.
