ABSTRACTInner cell masses (ICMs) dissected from -day rat blastocysts were aggregated with -day mouse morulae. Successful aggregates formed blastocysts in vitro and morphologically normal -day conceptuses in the mouse uterus. Immunofluorescent analysis of these conceptuses revealed that rat cells were only present in the embryonic ectoderm and endoderm and never in the trophectoderm derivatives, although rat trophoblast did develop in the mouse uterus in various control experiments. The single-cell resolution of this technique extends the results obtained from aggregating mouse ICMs with mouse morulae and provides strong evidence that ICM cells, although not overtly differentiated, are determined by the blastocyst stage.