The FLT4 gene encodes a tyrosine kinase receptor related to the two identified receptors for vascular endothelial growth factor (VEGF), FLT1 and FLK1/KDR. Two isoforms of FLT4, differing by their C-terminal ends, have been identified. The long form has 65 additional amino acid residues. We have shown that FLT4 is a highly glycosylated, relatively stable, cell surface associated kinase of approximately 180 kDa. In order to study the signal transduction molecules associated with the FLT4 pathway, and in the absence of a known ligand, we constructed two chimeric molecules (FF4S and FF4L) made of the extracellular region of the CSF1 receptor (Fms gene product) and of the transmembrane and intracellular regions of either form of FLT4. These two chimeric forms were expressed in Rat 2 transfectants. We assayed the ligand-induced capacity of the FF4 short and long forms to sustain growth of Rat 2 cells in semisolid medium. In a soft agar assay, only the long form was able to induce the growth of Rat 2 cells upon ligand treatment. The two forms of FLT4 therefore have different functional capacities. We looked for association and/or phosphorylation of phospholipase C gamma (PLC gamma) and phosphatidylinositol-3'-phosphate (PI3K), after stimulation of the FF4 molecules by CSF1. Finally, we have studied the expression of the Flt4 gene in mouse embryos and in the adult by in situ hybridization. Flt4 transcripts were found at day 12.5 post-coïtum and thereafter, including the adult mouse, predominantly in the pericardium, pleural membranes and in the lung.