The bone morphogenetic protein (BMP) signaling pathway plays an essential role during gastrointestinal (GI) tract development in vertebrates. In the present study, we use an antibody that recognizes the phosphorylated and activated form of Smad1, 5, and 8 to examine (by immunohistochemistry) the endogenous patterns of BMP signaling pathway activation in the developing GI tract. We show that the endogenous BMP signaling pathway is activated in the mesoderm, the endoderm, and the enteric nervous system (ENS) of the developing chick GI tract and is more widespread than BMP ligand expression patterns. Using an avian-specific retroviral misexpression technique to activate or inhibit BMP signaling pathway activity in the mesoderm of the gut, we show that BMP activity is required for the pattern, the development, and the differentiation of all three tissue types of the gut: mesoderm (that forms the visceral smooth muscle), endoderm (that forms the epithelium), and ectoderm (that forms the ENS). These results demonstrate that BMP signaling is activated in all the tissue layers of the GI tract during the development and plays a role during interactions and reciprocal communications of these tissue layers. Developmental Dynamics 234:312–322, 2005. © 2005 Wiley-Liss, Inc.