The cdk1/cyclin B1 complex is a universal regulator known to be responsible for driving the cell-cycle from the G2- to the M-phase. To investigate the effects of irradiation on the activity of this complex in preimplantation embryos, we irradiated one- and two-cell mouse embryos with X-rays, and measured the fluctuations of histone H1 and cdk1 kinase activity. Four mouse strains with different radiation sensitivities were chosen: the BALB/c and the Heiligenberger (radiation-sensitive) and the C57BL and the CF1 (radiation-resistant) strains. Embryos irradiated in the first cell-cycle arrested in the G2-phase. However, the dynamics of this radiation-induced G2-block were different between the mouse strains tested. Indeed, in the C57BL and the CF1 strains, X-irradiation with 2.5 Gy induced a very short G2 block before the one-cell embryos could then proceed to mitosis. On the contrary, X-irradiation in BALB/c induced a G2-arrest that lasted about 20 h, with the percentage of embryos blocked in G2 depending on the dose, whilst in the Heiligenberger strain, all irradiated embryos developed a G2-block, which was dependent in duration on the radiation dose. In all mouse strains, the histone H1 kinase activity remained low during the G2 arrest, while it showed values comparable to that of control embryos during mitosis. X-irradiation is known to induce a change in the phosphorylation state of the cdk1 protein kinase in adult somatic cells. In embryos from the BALB/c and C57BL strains, the histone H1 kinase activities were confirmed by the cdk1 phosphorylation pattern: the inactive and phosphorylated form of cdk1 was observed in G2 arrested 1-cell embryos, while the active and dephosphorylated form of cdk1 was present in dividing control and irradiated 1-cell embryos. X-irradiation at the 2-cell stage only induced a short G2-arrest in all tested mouse strains. In conclusion, cell-cycle effects in early embryos under normal conditions and after irradiation are strictly paralleled by changes in the activity of the central cell-cycle driving enzyme complex.