Vertebrate precardiac mesoderm contains cells destined to become cardiomyocyte or endothelial cells. To determine the stability of these phenotypes freshly isolated embryonic day (E) 2.5–E6 chicken hearts were immunostained for myosin heavy chain (MyHC) to identify cardiomyocytes, and von Willebrand factor (vWF) and Flk-1 to identify endothelial cells. At E2.5–E3, 90% of cells express only MyHC and 6% express only vWF/Flk-1. However, 2% MyHC+ cells in E2.5–E3 hearts and 0.3% in E4–E6 hearts, also express vWF/Flk-1; and when cultured 3 days, >40% of the MyHC+ cells express vWF/Flk-1, but they do not express Vezf1, vascular endothelial cadherin, or Tie2. Thus, only a subset of endothelial genes are induced in cultured cardiomyocytes. While the subsequent developmental fate of embryonic heart cells exhibiting a vWF+/MyHC+ phenotype is unknown, analysis of this phenotype may provide information pertinent to mechanisms of cell phenotype stability, cellular transdifferentiation, and induction of stable cell types from embryonic stem cells. Developmental Dynamics 236:2512–2522, 2007. © 2007 Wiley-Liss, Inc.