The early embryonic heart consists of two cell types. The cells form an inner epithelial tube of endocardium within an outer tube of myocardium separated by a cell‐free extracellular matrix. A crucial process in heart development is the production of cushion mesenchyme in the atrioventricular (AV) canal and outflow tract (OT). Cushion mesenchyme differentiates from the endocardium in response to signaling molecules produced by the adjacent myocardium. In chicken hearts, both transforming growth factor‐β3 (TGF‐β3) and TGF‐β2 are present and have been identified as being important in the production of cushion mesenchyme. We were interested in how the signals from these two similar molecules may be differentiated during early heart development. To this end, we examined the expression of endoglin, a TGF‐β receptor molecule, in the developing chick heart. Endoglin is typically located on endothelial cell layers and binds tightly to TGF‐β1 and TGF‐β3 but not well to TGF‐β2. We show that during the formation of the primitive heart tube, endoglin is found at relatively high levels in both presumptive myocardium and endocardium. However, as myocardium differentiates and development proceeds, endoglin expression is progressively reduced. At stage 20 in the heart, endoglin expression is most readily seen in the AV canal and the OT. This pattern of expression is similar to the reported TGF‐β3 expression patterns in the heart. Dev. Dyn. 1998;213:237–247. © 1998 Wiley‐Liss, Inc.