GATA-4 knockout mice die by 9.5 days postcoitum and exhibit profound defects in ventral morphogenesis, including abnormal foregut formation and a failure of fusion of the bilateral myocardial primordia. During early mouse development, GATA-4 is expressed in cardiogenic splanchnic mesoderm and associated endoderm, suggesting that the presence of this transcription factor in one or both of these tissue types is essential for ventral development. To distinguish whether GATA-4 expression in mesoderm or endoderm accounts for the phenotype of the knockout mouse, we prepared chimeric mice by injectingGata4−/− ES cells into 8-cell stageROSA26(Gata4+/+) embryos. We identified a series of high percentage null chimeras (8–10 days postcoitum) in whichGata4+/+ cells were restricted to visceral yolk sac endoderm and small portions of the foregut/hindgut endoderm. Despite an absence of GATA-4 in all other cells of these embryos, there was normal development of the heart, foregut, and surrounding tissues. We conclude that expression of GATA-4 in endoderm rather than cardiogenic mesoderm is required for ventral morphogenesis.