It has been established that the quality of water for the preparation of culture media has an essential influence on the percentage of two cell-stage mouse embryos developing successfully until the blastocyst stage. The quality of media prepared with bidistilled water (p less than 0.001) is inferior to that prepared with bidestilled demineralized water (p less than 0.02). Best results were attained by using the medium prepared with Nanopure water (Barnstead), in which 73.2% of embryos developed into blastocysts. There was no statistically significant difference between this medium and the commercial liquid medium (Sigma), in which 83.1% of embryos developed into blastocysts. The age of the medium and the percentage of added serum have no statistically significant influence on the outcome of the test. Contact of the medium with the syringe containing a black rubber piston proved explicitly embryotoxic (p less than 0.001).