During mammalian embryonic development, the formation and subsequent expansion of a fluid-filled cavity, the blastocoel, is crucial for successful implantation. Our present experiments were aimed at exploring the contribution of Rho-kinase, a downstream effector of the small GTP-binding protein RhoA, to mouse blastocoel formation. RT-PCR analysis showed that Rho-kinase mRNA is present throughout mouse preimplantation development. When 2-cell embryos were cultured in the presence of a specific inhibitor of Rho-kinase, Y-27632, they developed to the morula stage but failed to develop to the blastocyst stage. Y-27632 inhibited the formation of the blastocoel cavity from the morula stage, and this inhibitory effect was reversible when embryos were returned to medium without Y-27632. Moreover, Y-27632 reduced the rate of re-expansion of blastocysts collapsed by cytochalasin D upon transfer to the control medium. These results suggest that Rho-kinase is likely involved in blastocyst formation.