The nuclei of preimplantation mouse embryos were identified after labelling with either DAPI or Hoechst 33258. During the 4- and 8-cell stages the peripherally located nuclei become clustered nearer the centre of the embryo. This nuclear migration towards the base of each cell was also observed during the development of couplets of 2/4 and 2/8 cells. Most blastomeres isolated from compact 8-cell embryos contained a nucleus located in their basal half away from the microvillous pole. The displacement may be critical for the generation of a range of sizes of blastomeres in the 16-cell embryo.