We present the partial cloning and the expression patterns of two putative growth factor receptor molecules named Quek1 and Quek2 (for quail endothelial kinase) in chick and quail embryos from gastrulation to embryonic day 9 (E9). Quek1 and Quek2 show high homology to three interrelated murine and human genes, flk-1, KDR and flt. Flt was recently shown to be the receptor for the endothelial cell mitogen vascular endothelial growth factor (VEGF). In situ hybridization of Quek1 and Quek2 to sections of avian embryos showed that they are both expressed essentially by endothelial cells, that we identified with a monoclonal antibody (Mab) QH1 specific for endothelial and white blood cells of the quail. Quek1 is expressed in the mesoderm from the onset of gastrulation, whereas Quek2 message is first detected on QH1-expressing endothelial cells. The expression pattern of Quek1 suggests that it could identify the putative precursor of both endothelial and hematopoietic lineages, the hemangioblast. Quek1 and Quek2 are not expressed in all endothelial cells throughout life. At E9, after the initial phase of vasculogenesis, these genes are switched off in various compartments of the vascular network.