Potassium channels play important roles in many cellular processes, including cell-cycle progression and cell differentiation. In the present study, we investigated the pattern of expression of the mouse ether-à-go-go-related (KCNH2; MERG1A) potassium channel during mouse embryogenic development. Analysis by reverse transcription-polymerase chain reaction revealed maternal MERG1A transcripts until the late 2-cell stage of development, after which MERG1A expression from the zygotic genome was low until the 8-cell stage, then rose in the morula, but was low in trophoblast compared to inner cell mass cells. A trophoblast stem cell line also was shown to express MERG1A mRNA. Immunoblotting of oocytes, blastocysts, and the trophoblast stem cell line revealed different posttranslationally processed forms of MERG1A. Immunofluorescence analysis showed that the subcellular localization of MERG1A varied at different stages of the embryogenic cell cycle. In addition, MERG1A protein levels increased following compaction at the 8-cell stage, and its distribution became polarized. This relocalization of MERG1A was affected by treatment with specific inhibitors of ether-à-go-go-related gene (ERG)-channel function and of actin polymerization. Puromycin treatment of morulae indicated that membrane-associated MERG1A had a half-life of greater than 24 h. The ERG-specific inhibitor E-4031 reduced the incidence of blastocyst formation and the number of cells per blastocyst. These results show that MERG1A is developmentally regulated and suggest that it might play a role in early mouse embryogenic development.