The presence of metastases in regional lymph nodes is a strong indicator of poor patient survival. A number of clinical and experimental studies suggest that tumor-induced lymphangiogenesis driven by vascular endothelial growth factor (VEGF)-C- and/or VEGF-D-induced activation of VEGF receptor (VEGFR)-3 may promote metastasis to regional lymph nodes. Here we show that constitutive VEGF-C and VEGF-D expression by tumor cells of diverse origin grown in tissue culture does not correlate with metastatic potential in vivo. However, tumors derived from cell lines that do not constitutively express VEGF-C or VEGF-D in tissue culture can nevertheless express one or both of these factors. We demonstrate that both tumor and stromal cells can contribute to this expression, suggesting that tumor cell-host interactions determine tumor expression of VEGF-C and VEGF-D. Using immunocompetent rat mammary tumor models, we show in two ways that this expression can promote metastasis via the lymphatics. Firstly, ectopic expression of a soluble VEGFR-3 receptor globulin protein in MT-450 tumor cells that are highly metastatic via the lymphatics blocked VEGF-C and VEGF-D activity and suppressed metastasis formation in both the regional lymph nodes and the lungs. Secondly, ectopic expression in the weakly metastatic NM-081 cell line of a mutant form of VEGF-C that is only able to activate VEGFR-3 strongly promoted metastasis of these cells to the regional lymph nodes and lung. These data show that expression of VEGF-C and VEGF-D in tissue culture does not reflect expression in vivo and that activation of VEGFR-3 in the absence of VEGFR-2 activation is sufficient to promote tumor-induced lymphangiogenesis and metastasis, and they support the notion that blockade of VEGFR-3 activation will be useful as a novel form of cancer therapy.