The non-cellular investments were damaged in only 2.8% of rabbit morulae frozen/ thawed in plastic straws. Similar results (4.8%) were obtained with glass ampoules only when morulae were slowly frozen and slowly thawed in siliconized ampoules. Freezing/thawing by other methods and the use of untreated ampoules resulted in high incidence of embryos with damaged investments (41-61%). Morulae frozen/thawed by a method allowing an almost complete elimination of investment injury displayed high survival when slowly frozen and rapidly thawed (90.7%) while slow freezing and slow thawing decreased survival (74.5%). Morulae subjected to rapid freezing were for the first time successfully stored at--196 degrees C. After rapid thawing 59.6% morulae survived. The mechanism of damage to the non-cellular investments of rabbit embryos is discussed.