Preinvasive mammary pathologies in humans and rat chemical carcinogenesis model systems have an increased microvascular density relative to normal tissue. This suggests the possibility of preventing invasive breast cancer by inhibiting angiogenesis. Vascular endothelial cell growth factor (VEGF) is a potent angiogenic growth factor, commonly involved in tumor-induced angiogenesis. Here, we show that both VEGF and VEGFR2 expression increase with histological progression to invasive disease in the rat 7,12-dimethylbenz [a]anthracene (DMBA) model. Other VEGF receptors, VEGFR1, neuropilin 1 and neuropilin 2, are constitutively expressed throughout progression. To examine whether VEGF signaling is functionally relevant to tumor-induced endothelial tubule formation in vitro and for tumor formation in vivo, we utilized the VEGFR2 inhibitor, ZD6474. In vitro endothelial cell tubulogenesis induced by isolated mammary organoids or carcinoma in situ from DMBA-treated rats is inhibited by ZD6474, in a dose-dependent fashion. The administration of ZD6474 to DMBAtreated rats inhibits the formation of atypical ductal hyperplasia and carcinoma in situ by greater than 95% (Po0.05), when administered 1 week or 6 weeks post-DMBA initiation. Invasive disease was absent in all ZD6474 cohorts. These data support the hypothesis that progression of DMBA-induced preinvasive mammary pathologies to palpable disease requires angiogenesis via a VEGF-dependent mechanism. Keywords: ZD6474; VEGF; VEGFR2; DMBA; chemoprevention Angiogenesis is the process of forming new vessels from pre-existing vessels, 1 and has been shown to be important in the process of breast cancer progression and metastasis. 2 We and others have shown that angiogenesis is also important in the process of breast cancer formation, as indicated by the stepwise increase in microvascular density with progression from normal epithelium to invasive disease. [3][4][5][6] In addition, we have shown that in the rat model system of DMBA-induced mammary tumor formation there is a similar pattern of upregulated microvascular density with progression. 7 Finally, if one isolates normal epithelium or tumor tissue from DMBA-treated rats, both are capable of inducing endothelial tubule formation in vitro, indicating that the angiogenic potential of these tissues is greater than that from vehicle-treated control rats. 7