The objective of this study was to compare the efficiency of 2 media on the vitrification of mouse compacted morulae, early blastocysts and expanded blastocysts after equilibration at room temperature or 4 degrees C. Embryos were equilibrated for 10 min in either 25% VS3 (Rall Equilibration Medium,REM) or 10% glycerol + 20% propylene glycol (Massip Equilibration Medium, MEM) in DPBS at 20 degrees C or 4 degrees C. For vitrification either 100% VS3 (Rall Vitrification Medium RVM) or 25% glycerol + 25% propylene glycol (Massip Vitrification Medium, MVM) in DPBS was used. Embryos equilibrated at room temperature were loaded in 20 mu L of vitrification media into 250 mu L straws and then immediately (30 sec) plunged into liquid nitrogen (LN2). Alter equilibration at 4 degrees C the embryos were put into straws with 20 mu L of precooled vitrification medium and after 20 min at 4 degrees C they were plunged-into LN2. Embryos from both groups were thawed in a 20 degrees C water-bath for 20 sec, transferred to 1.0 M sucrose in DPBS for 5 min and then cultured for 24 to 48 h in Whitten's medium at 37 degrees C in 5% CO2 in air. In the groups of embryos prepared for vitrification at room temperature the survival rate of compact morulae vitrified in RVM was higher than those vitrified in MVM (65/70, 93% vs 49/74, 66%; P<0.01). No difference was found in the survival rate of early blastocysts and expanded blastocysts vitrified in RVM or MVM (30/83, 36% vs 25/75, 33% and 4/66, 6% vs 4/76, 5%). No difference was found between the survival rate of compact morulae after equilibration with RVM or MVM at 4 degrees C (62/75, 83% vs 52/74, 70%). Both the early blastocysts and expanded blastocysts equilibrated at 4 degrees C MVM yielded a higher survival rate than RVM (28/74, 38% and 40/70, 57% vs 4/75, 5% and 4/77, 5%; P<0.01). We conclude that, of the 3 developmental stages, compact morulae withstand the vitrification process best, and reduction of the temperature prior to plunging into. LN2 is not required. A 10-fold increase in the survival rate of expanded blastocysts can be achieved using low temperature equilibration (4 degrees C) and MVM. (C) 1999 by Elsevier Science Inc.