Studies with intact preimplantation mouse embryos and some types of chimaeric aggregates have shown that the most advanced cells are preferentially allocated to the inner cell mass (ICM) rather than the trophectoderm. Thus, differences between 4-cell and 8-cell stage embryos could contribute to the tendency for tetraploid cells to colonise the trophectoderm more readily than the ICM in 4-cell tetraploid[harr ]8 cell diploid chimaeras. The aim of the present study was to test whether 4-cell stage embryos in 4-cell diploid[harr ]8-cell diploid aggregates contributed equally to all lineages present in the E12.5 conceptus. These chimaeras were compared with those produced from standard aggregates of two whole 8-cell embryos and aggregates of half an 8-cell embryo with a whole 8-cell embryo. As expected, the overall contribution of 4-cell embryos was lower than that of 8-cell embryos and similar to that of half 8-cell stage embryos. In the 4-cell[harr ]8-cell chimaeras the 4-cell stage embryos did not contribute more to the trophectoderm than the ICM derivatives. Thus, differences between 4-cell and 8-cell embryos cannot explain the restricted tissue distribution of tetraploid cells previously reported for 4-cell tetraploid[harr ]8-cell diploid chimaeras. It is suggested that cells from the more advanced embryo are more likely to contribute to the ICM but, for technical reasons, are prevented from doing so in simple aggregates of equal numbers of whole 4-cell and whole 8-cell stage embryos.