The results of experiments in which horseradish peroxidase (HRP) was used to mark single trophectoderm or inner cell mass (ICM) cells in situ in mouse blastocysts have led to the proposal that growth of the trophectoderm depends on stem cells located in the inner cell mass. Thus, the finding that the visual centre of clones formed following labelling of the central polar trophectoderm cell in early or expanding blasrtocysts was consistently shifted towards or into the mural trophectoderm was attributed to their displacement by ICM-derived cells. However, the frequency with which central polar cells were displaced is likely to have been overestimated by using the visual centre of descendant clones as the index of their location. also, the possibility that displacement of central polar cells was an artefact of the marked temporary interruption of their cycling that resulted from labelling was not discounted. Furthermore, no attempt was made to ascertain whether cells located elsewhere in the polar trophectoderm also moved murally, as expected if there is a general displacement of such cells. In the present study, labelling of either the central or a peripheral polar trophectoderm cell with HRP was achieved without obviously perturbing their subsequent proliferation. Moreover, displacement was assessed by recording the location of the proximal boundary rather than the visual centre of the resulting clones. Even by this conservative criterion, the majority of labelled central cells moved towards or into the mural trophecto derm. In marked contrast, however, labelled peripheral polar cells moved murally in only a minority of cases. The remainder either retained their original position or moved towards rather than away from the central polar region. Such an anisotropic pattern of growth of the polar trophec toderm is not readily explicable in terms of recruitment of cells from the ICM. Rather, it accords with the view that the polar trophectoderm is a proliferative centre, and suggests that movement murally of its surplus cells may be restricted circuinferentially, possibly through anchorage of the junctional trophectoderm cells that extend processes over the free surface of the ICM.