Vasculogenesis, the de novo formation of embryonic blood vessels from their angioblastic precursors in situ, is supposed to be under the control of polypeptide growth factors and their receptors. The receptor tyrosine kinase flk-1 and its high-affinity ligand vascular endothelial growth factor (VEGF) represent an endothelial specific signal transduction system expressed during embryonic vascular growth in the mouse. We have cloned the quail homologs of VEGF and flk-1 using PCR and have investigated their expression pattern in vivo. As shown by Northern analysis and reverse transcription PCR, VEGF and flk-1 mRNA (3.9 and 5.8 kb, respectively) were already present in the unincubated blastodisc at low levels and were largely upregulated during gastrulation at Embryonic Day 1. As detected by in situ hybridization, flk-1 mRNA was initially present in the entire mesoderm of Day 1 embryos but from Day 2 on was restricted to endothelial cells. At Day 2 VEGF was ubiquitously expressed in the embryo proper and was mainly restricted to the vascularized part (area vasculosa) in the yolk sac. Later on VEGF expression was detected in all organs. In the kidney VEGF mRNA was mainly localized to the glomeruli. This pattern of expression is consistent with the pattern found during mouse embryogenesis. We have recently established an in vitro model of vasculogenesis in which hemangioblastic precursors are induced in cell cultures from the unincubated quail blastodisc by basic fibroblast growth factor (bFGF) and give rise to blood vessels in vitro. Taking advantage of this in vitro model we examined whether FGF and VEGF act in concert during vasculogenesis. We found that the flk-1 receptor mRNA is dramatically upregulated within 24 hr upon the addition of FGF to quail blastodisc cell cultures. This inducibility in response to FGF is confined to the first 24 hr of culture. The early expression of the flk-1 mRNA may characterize the differentiation of hemangioblastic precursors from pluripotent epiblast cells which in vivo is initiated during gastrulation. Thus, the time course and the pattern of expression during embryogenesis in different species suggest a major role for the VEGF/flk-1 signal transduction system in vasculogenesis and angiogenesis.