Based on the criteria of relative size and cell surface polarization, subpopulations of outer (larger, polar) and inner (smaller, apolar) blastomeres have been isolated from mouse 16-cell morulae and reaggregated in groups of 16 cells, and the developmental potential of the aggregates has been assessed both in vitro and in vivo. Aggregates of outer, inner, or mixed groups of cells all formed blastocysts which outgrew and contained both characteristic trophectoderm and alkaline phosphatase positive inner cell masses in vitro. However, significant differences were observed in the timing of blastocyst formation, depending on cell type. Aggregates of outer cells recompacted more slowly, commenced fluid accumulation earlier, and contained more cells at the blastocyst stage than did inner cell aggregates. Aggregates containing both outer and inner cells were intermediate. Postimplantation development of blastocysts derived from aggregates of outer or inner cells, after transfer to pseudopregnant recipients, was normal and comparable to zona-intact control embryos. This relationship between the expression of different morphological and behavioural properties by the precursor cells of the trophectoderm and inner cell mass and the existence of totipotent cells in both outer and inner subpopulations is discussed.