Exit from M-phase and completion of cell division requires inactivation of M-phase promoting factor (MPF), a heterodimer composed of the regulatory cyclin B1 and the catalytic p34cdc2 kinase. Inactivation of MPF is associated with cyclin B1 degradation that is brought about by the ubiquitin-proteasome pathway. Our study examined the role of the proteasome in the first mitosis of rat embryos and its participation in the regulation of cyclin B1 degradation and MPF inactivation. We show that in the early zygote the proteasome is evenly distributed in the ooplasm and the nucleus, whereas during mitosis it accumulates on the spindle apparatus. We further demonstrate that inhibition of proteasomal catalytic activity prevents 1-cell embryos from undergoing mitosis. This mitotic arrest is associated with the presence of relatively high amounts of cyclin B1, which unexpectedly does not result in elevated MPF activity. Our findings strongly imply that completion of the first embryonic division depends on proteasomal degradation and that cyclin B1 is included among its target proteins. They also provide the first evidence that MPF inactivation at this stage of development is not solely dependent upon cyclin B1 degradation and is insufficient to allow the formation of the 2-cell embryo.