During development of the heart, the endocardium of the developing cushion tissue transforms into mesenchymal cells that rapidly invade the underlying extracellular matrix. This invasive process, along with subsequent proliferation and differentiation of the mesenchyme, are essential for the normal formation of valvular and septal structures. Several factors have been identified that appear to initiate and/or control this transformation process, including the growth factor TGF-beta. In these studies we have investigated whether hepatocyte growth factor/scatter factor (HGF/SF) may also be involved in some aspects of this transformation process. Using an immunohistochemical approach we have detected a spatially restricted localization of HGF/SF to the myocardial cells of the cushion tissue. HGF was detected in extracts of the developing heart, and the presence of the active form correlated with the onset of the transformation process and the elevation of urokinase activity. The endocardial-derived mesenchymal cells of the cushion tissue were found to express the c-met HGF receptor. Isolated endocardial cells responded to the addition of HGF with increases in motility, proliferation, and urokinase production. The results from these studies suggest that HGF may function as a myocardial-derived mediator of the epithelial-mesenchymal transformation by inducing and/or maintaining the mesenchymal cell phenotype. The increase in urokinase expression by the cushion tissue cells, in response to HGF, may facilitate the invasive abilities of these cells and also provide a means of maintaining high levels of active HGF required for the stimulation of cell proliferation and migration.