Compaction is associated with major changes in the transport processes in preimplantation embryos. Before this time the processes are homocellular, in which all the component cells exchange materials across their cell membranes with a common environment. After compaction the outer trophoblast cells become organized into a simple, squamous epithelium which is capable of transcellular vectorial transport, that selectively controls the movement of materials into the embryo. Measurements of the intracellular concentrations and membrane permeabilities of Na+, K+ and Cl- in the mouse oocyte and two-cell embryo have demonstrated that they undergo significant changes during this period of development. The development of transcellular transport across the trophectoderm is fundamental in the regionalization of the embryo. These physiological mechanisms are involved in the formation of the blastocoele fluid, and may be dependent on the development of regionally located Na+, K+-ATPase on the juxtacoelic surfaces of the trophoblast cells.