Previously, we demonstrated the significance of vascular endothelial growth factor (VEGF) in promoting the growth of tetracycline-regulated human VEGF165 retroviral vector transduced T47-D breast carcinoma cells, particularly at the early stages of tumor development (Cancer Res. 57 (1997) 3924). Here, we showed histologically that the VEGF overexpressing (VEGF (+)) T47-D cells formed a distinct tumor nodule at day 11, while control cells showed no evidence of replication. The VEGF (+) tumors contained large avascular cavities at days 11 and 21, which were replaced by basement membrane-lined channels at day 30. The number of proliferating tumor cells was not significantly different between the VEGF (+) and control tumors, but the number of apoptotic cells was significantly decreased in the VEGF (+) tumors. Increased nitric oxide synthase (NOS) activity was also observed in the VEGF (+) tumors. These findings indicate that VEGF contributes to tumor growth through inhibition of apoptosis and increased NOS activity, which may be critical during pre-vascular stages of tumor development.