It has been demonstrated that embryonic stem cells form adhesive contacts with external blastomeres of mouse morula, while there is no such contact with blastocysts. The development of morula in the blastocysts is delayed inside a dense layer of such cells; however, in some cases, external blastomeres of the morula begin to differentiate into trophoblastic cells. The introduction of an excessive number of embryonic stem cells (15-20) into a 4-8-cell embryo results in abnormal development. When heterotypic embryonic mink stem cells are co-cultivated, they show only very weak adhesion with mouse blastomeres and are displaced as a result of compactization. When blastocysts are formed after the injection of heterotypic embryonic stem cells, such cells remain in the perivitelline space. In some cases, heterotypic embryonic stem cells continue to be determined in the trophoblastic direction and produce trophoblastic vesicles autonomously. The role of cell interaction in the determination of cells during early mammalian development is discussed.