SummaryEmbryos of good to excellent morphology were recovered 6.5 to 7 days after ovulation, washed in Dulbecco's phosphate‐buffered saline plus 10 per cent heat‐treated foetal calf serum (PBS), and placed in PBS plus 5 per cent 1, 2 propanediol for 5 mins followed by PBS plus 10 per cent 1, 2 propanediol for 10 to 20 mins. The embryos were then placed into 0.5 ml French straws, cooled initially to −6°C at 4°C/min, seeded and cooled further from −6°C to −33°C at a rate of either 0.3°C/min or 0.8°C/min. When straws reached −33°C they were plunged into liquid nitrogen. Within each cooling treatment, straws were thawed either in a 37°C water bath for 20 sees, or at ambient temperature in air. 1, 2 propanediol was removed in four steps, each step of 6 mins duration at room temperature. The thawed embryos were then cultured in Ham's F10 plus 10 per cent FCS in a 5 per cent CO2 incubator at 38°C. After 48 h, the morphological appearance of the embryos was evaluated on a scale of 1 to 5 (1 = excellent, 5 = severely degenerate). Four embryos measuring 130 to 175 μm in diameter, 23 embryos measuring 200 to 945 μm and three embryos larger than 1000 μm were included in the experiment. All four of the small embryos showed good morphology at the end of the culture period (scores of 1.5 to 3.5) whereas none of the embryos larger than 1000 μm in diameter survived cryopreservation (all scores 5.0). Post‐treatment scores for the 23 embryos in the middle‐sized group (220 to 945 μm) ranged from 1.5 to 5.0. Rapid thawing was superior to slow thawing (p<0.07) for embryo quality after 48 h of culture, although the rapidly thawed embryos appeared to be more damaged immediately after thawing as determined by cracked capsules or zonae pellucidae, or embryos breaking into pieces. The rate of cooling had no significant effect on any response.