Vascular endothelial growth factor (VEGF), in addition to being a specific mitogen of endothelial cells in vitro, is also known to induce angiogenesis in vivo. These functions suggest that VEGF may play an important role in the growth of hemangiosarcomas. Previous studies have demonstrated the expression of VEGF and its receptors, flt-1 or KDR/flk-1, in hemangiosarcomas by immunohistochemical staining and in situ hybridization. In the present study, however, we demonstrated that tumor cells of the hemangiosarcoma cell line ISO-HAS express mRNA of VEGF and its two receptors, flt-1 and KDR, and secrete VEGF protein. VEGF mRNA expression and protein secretion were found to be enhanced by phorbol 12-myristate 13-acetate. In addition, we demonstrated that ISO-HAS cells respond to recombinant human VEGF165 with a dose-dependent up-regulation of cell proliferation and growth. These results suggest that the VEGF-VEGF receptor system plays a role in proliferation and growth of hemangiosarcoma cells.