There are reports of the storage at O-lOoC of fertilized rabbit (Chang 1947, 1948; Hafez 1965) and sheep ova (Averill 1956; Kardymowicz et al. 1963; Kardy-mowicz, Kardymowicz, and Grochowalski 1966; Kardymowicz, Kardymowicz, and Kremer 1966). However, the storage of fertilized mouse ova in vitro does not appear to have been studied. Wales (1969) found that glycolysis at 37�C by cultured blastocysts was not impaired if the two-cell embryo was stored at 5�C for a short period prior to culture, and the author suggested that storage of two-cell embryos at this temperature may be successful. In the present study the viability of mouse embryos after various periods at low temperature was investigated.