The survival after transfer of frozen-thawed mouse blastocysts obtained from culture of in-vitro fertilized oocytes or 1- and 2-cell ova was compared. About 10% of transferred embryos developed to term in each group and there was no difference between embryos fertilized in vitro or in vivo. In addition to embryonic loss due to transfer, in-vitro cultivation and freezing reduced the proportion of fetuses considered viable at Day 15 of pregnancy (29.8 versus 50.7% and 26.3 versus 50.7% respectively). When used together these procedures had an additive effect on fetal wastage (18.4 versus 50.7%). In-vitro culture also entailed a significant increase of resorbing implantation sites (10.2 versus 4.3%). The re-expansion rate after freezing and thawing of blastocysts grown in vitro was paradoxically greater than that of blastocysts grown in vivo (85.8 versus 54.6%).