Solid tumor growth is dependent upon angiogenesis, a process by which soluble factors released from a tumor induce the sprouting and growth of new blood vessels from nearby venules into the tumor. This process of tumor vascularization provides tumor cells with nutrients, oxygen, and an enhanced ability to establish metastasis at peripheral sites by migration through the circulatory system. Vascular endothelial growth factor is a potent angiogenic factor that is expressed at low levels by most normal cells, can be upregulated in normal cells by exposure to hypoxia or phorbol esters, and exhibits high levels of constitutive expression in some human tumors and tumor cell lines. The mechanism underlying the stable change that results in VEGF overexpression in tumors is unknown. Here, we demonstrate that both hypoxia and TPA induce stabilization of VEGF mRNA, that stabilization by hypoxia is rapidly reversible upon reexposure to normoxia, and that tumor cell lines exhibiting constitutive overexpression of VEGF also exhibit constitutive stabilization of VEGF transcripts. Stabilized VEGF transcripts in tumor cells are refractile or nearly refractile toward further stabilization by TPA or hypoxia, respectively. Furthermore, cycloheximide induces stabilization of VEGF mRNA in normal cells but has no effect on VEGF transcript stability in tumor cells that contain stabilized transcripts. These results suggest that normal signal transduction mechanisms mediate stabilization of the VEGF mRNA, and that mutations in this regulatory pathway in tumor cells may lead to chronic message stabilization, overexpression of VEGF proteins, and ensuing tumor vascularization.