Various factors affect the viability of embryos after freezing and thawing. When mouse embryos are supercooled below -6 degrees C before ice induction (seeding), survival is markedly reduced, apparently by inadequate dehydration of the embryos during subsequent cooling. Immediately after thawing, eight-cell mouse embryos and blastocysts experience a delay before normal embryonic development is resumed. A restorative period in culture combined with a modification of the synchrony between embryo and recipient helps to maximize survival following transfer. No loss of viability was observed in eight-cell mouse embryos after storage at -196 degrees C for four years. The preservation of the embryos of laboratory species other than the mouse and rabbit is limited by our lack of knowledge of the culture requirements for the development of such embryos.