The transcription factor Oct-4 is a marker of pluripotency in mouse and human embryonic stem (ES) cells. Previous studies using a tetracycline-regulated Oct-4 transgene in the ZHBTc4 cell line demonstrated that downregulation of Oct-4 expression induced dedifferentiation into trophoblast, a lineage mouse ES cells do not normally generate. We found that transfection of Oct-4-specific short interfering RNA significantly reduced expression and functional activity of Oct-4 in mouse and human ES cells, enabling its role to be compared in both cell types. In mouse ES cells, Oct-4 knockdown produced a pattern of morphological differentiation and increase in expression of the trophoblast-associated transcription factor Cdx2, similar to that triggered by suppressing the Oct-4 transgene in the ZHBTc4 cell line. In addition, downregulation of Oct-4 was accompanied by increased expression of the endoderm-associated genes Gata6 and α-fetoprotein, and a gene trap associated with primitive liver/yolk sac differentiation. In human ES cells, Oct-4 knockdown also induced morphological differentiation coincident with the upregulation of Gata6. The induction of Cdx2 and other trophoblast-associated genes, however, was dependent on the culture conditions. These results establish the general requirement for Oct-4 in maintaining pluripotency in ES cells. Moreover, the upregulation of endoderm-associated markers in both mouse and human ES cells points to overlap between development of trophoblast and endoderm differentiation.