Survival rates on thawing were assessed for ultrarapidly frozen mouse oocytes following the removal of zonae pellucidae using an acid Tyrode's solution, pronase or a mechanical dissection technique. Significantly higher rates of survival were observed for zona-free oocytes than for zona-intact control oocytes (303/684, 44% v. 130/498, 26%; P < 0.001). The rates of survival observed for pronuclear stage embryos (72-76%) were much greater than those observed for oocytes and were not influenced by zona removal techniques. Parthenogenetic activation of oocytes by exposure to a 7% (v/v) ethanol solution was also shown to increase survival rates of ultrarapidly frozen oocytes (155/185, 84% v. 19/102, 19%) indicating that fusion of sperm to the plasma membrane or formation of a male pronucleus are not directly responsible for the increased survival rates of pronuclear stage embryos compared with oocytes. These data support the hypothesis that increased survival of ultrarapidly frozen pronuclear stage embryos is the result of changes to the plasma membrane and/or to the zona pellucida that occur following fertilization.