The usefulness of ultrarapid freezing of mouse pronucleate ova was investigated in comparison with slow, programmed freezing. Pronucleate mouse ova were frozen using an ultrarapid method in either 3.5 M dimethylsulphoxide (DMSO), 3.5 M propanediol (PROH) or a 1:1 mixture of both. After a brief exposure to the cryoprotectant, they were plunged into liquid nitrogen. Thawing was done at 37 degrees C and the cryoprotectant was rapidly diluted in a sucrose solution. Pronucleate ova were slowly cooled in a biological freezer using 1.5 M PROH as cryoprotectant. Thawing was done at room temperature and PROH was removed by multi-step dilutions. As control groups, pronucleate ova were either given no treatment, or exposed to PROH or DMSO without freezing and cultured in vitro. Ultrarapid freezing using 3.5 M DMSO as cryoprotectant resulted in rates of survival, cleavage and post-thaw development to blastocysts of 85, 89 and 37%, respectively. PROH as cryoprotectant, however, was inadequate in the ultrarapid freezing protocol and the combination of DMSO and PROH showed no further improvement. Slow, programmed freezing (using PROH) compared to ultrarapid freezing (using DMSO) resulted in similar rates of survival, cleavage and development of 80, 84, 20% and 92, 81 and 23% respectively. In conclusion, DMSO is a better cryoprotectant than PROH for ultrarapid freezing of pronucleate mouse ova, and ultrarapid freezing with 3.5 M DMSO is as effective as slow freezing with 1.5 M PROH, as evaluated by their subsequent development in vitro.