The effect of polyvinyl alcohol (PVA) on the development of mouse preimplantation embryos was examined. In Hoppe and Pitts medium supplemented with 0.1 or 1.0 mg/ml PVA, a high percentage (comparable to BSA-supplemented medium) of mouse 1-cell embryos developed to the expanded blastocyst stage. However, in medium supplemented with 10.0 mg/ml PVA, no embryos developed to the blastocyst stage. These results indicate that, at optimum concentration, PVA supports embryo development from the 1-cell to the expanded blastocyst stage, and that the PVA-supplemented medium is a valuable chemically defined medium for mouse embryonic culture. Also, in this study, in the PVA-supplemented medium, percentages and speeds of embryo development from each stage (1-cell, 2-cell, 8-cell and early blastocyst) to the expanded blastocyst stage were almost the same as those in the BSA-supplemented medium. These results suggest that these embryos developed to expanded blastocysts via normal processes, and that PVA supports embryo development in each stage up to the expanded blastocyst stage. Incidence of partial hatching and complete hatching of blastocysts was clearly decreased in cultures of each embryo from 1-cell to the early blastocyst stage in the PVA-supplemented medium. It has been considered that protease may participate in the hatching process of blastocysts in vitro, thus, it is probable that the low hatching rate of blastocysts in the PVA-supplemented medium is due to a decline in protease synthesis and/or secretion.