During cardiovascular development, fluid shear stress patterns change dramatically due to extensive remodeling. This biomechanical force has been shown to drive gene expression in endothelial cells and, consequently, is considered to play a role in cardiovascular development. The mechanism by which endothelial cells sense shear stress is still unidentified. In this study, we postulate that primary cilia function as fluid shear stress sensors of endothelial cells. Such a function already has been attributed to primary cilia on epithelial cells of the adult kidney and of Hensen's node in the embryo where they transduce mechanical signals into an intracellular Ca2+ signaling response. Recently, primary cilia were observed on human umbilical vein endothelial cells. These primary cilia disassembled when subjected to high shear stress levels. Whereas endocardial–endothelial cells have been reported to be more shear responsive than endothelial cells, cilia are not detected, thus far, on endocardial cells. In the present study, we use field emission scanning electron microscopy to show shear stress-related regional differences in cell protrusions within the cardiovasculature of the developing chicken. Furthermore, we identify one of these cell protrusions as a monocilium with monoclonal antibodies against acetylated and detyrosinated alpha-tubulin. The distribution pattern of the monocilia was compared to the chicken embryonic expression pattern of the high shear stress marker Krüppel-like factor-2. We demonstrate the presence of monocilia on endocardial–endothelial cells in areas of low shear stress and postulate that they are immotile primary cilia, which function as fluid shear stress sensors. Developmental Dynamics 235:19–28, 2006. © 2005 Wiley-Liss, Inc.