Casein kinase 2 (CK2) is a highly conserved, ubiquitously expressed serine/threonine protein kinase with hundreds of substrates. The role of CK2 in the G/M transition of oocytes, zygotes, and 2-cell embryos was studied in mouse by enzyme activity inhibition using the specific inhibitor 4, 5, 6, 7-tetrabromobenzotriazole (TBB). Zygotes and 2-cell embryos were arrested at G phase by TBB treatment, and DNA damage was increased in the female pronucleus of arrested zygotes. Further developmental ability of arrested zygotes was reduced, but that of arrested 2-cell embryos was not affected after releasing from inhibition. By contrast, the G/M transition in oocytes was not affected by TBB. These results indicate that CK2 activity is essential for mitotic G/M transition in early embryos but not for meiotic G/M transition in oocytes.