Studies have shown that some electrolytes, including Na.sup.+ and K.sup.+, play important roles in embryonic development. However, these studies evaluated these electrolytes by using inhibitors or knockout mice, with no mention on the changes in the intracellular electrolyte concentrations during embryogenesis. In this study, we used the electrolyte indicators CoroNa Green AM and ION Potassium Green-2 AM to directly visualise intracellular concentrations of Na.sup.+ and K.sup.+, respectively, at each embryonic developmental stage in mouse embryos. We directly observed intracellular electrolyte concentrations at the morula, blastocyst, and hatching stages. Our results revealed dynamic changes in intracellular electrolyte concentrations; we found that the intracellular Na.sup.+ concentration decreased, while K.sup.+ concentration increased during blastocoel formation. The degree of change in intensity in response to ouabain, an inhibitor of Na.sup.+ /K.sup.+ ATPase, was considered to correspond to the degree of Na.sup.+ /K.sup.+ ATPase activity at each developmental stage. Additionally, after the blastocyst stage, trophectoderm cells in direct contact with the blastocoel showed higher K.sup.+ concentrations than in direct contact with inner cell mass, indicating that Na.sup.+ /K.sup.+ ATPase activity differs depending on the location in the trophectoderm. This is the first study to use CoroNa Green AM and ION Potassium Green-2 AM in mouse embryos and visualise electrolytes during embryonic development. The changes in electrolyte concentration observed in this study were consistent with the activity of Na.sup.+ /K.sup.+ ATPase reported previously, and it was possible to image more detailed electrolyte behaviour in embryo cells. This method can be used to improve the understanding of cell physiology and is useful for future embryonic development studies.