Omentum has been used clinically to promote wound healing and to stimulate the revascularization of ischemic tissues. The biologic mechanism responsible for these effects has, however, not yet been defined. A number of polypeptide growth factors that possess potent angiogenic properties have recently been identified, and we therefore sought to determine whether one of these growth factors might be responsible for the angiogenic properties of the omentum. The levels of vascular endothelial growth factor (VEGF) protein in a number of rat tissues and organs were analyzed by Western and enzyme immunoassay analysis. Because omentum was found to have the greatest VEGF concentrations of the tissues examined, antibody neutralization, transcription inhibition assays, and Northern blot analysis were performed under hypoxic and normoxic conditions on tissues extractions and primary tissue cultures of omentum to further characterize the functional significance of VEGF expression in these tissues. The omentum demonstrated the highest VEGF secretion rate as well as the highest concentration of VEGF protein of the various rat tissues and organs examined. Fractionation studies of the omentum furthermore demonstrated that omental adipocytes, rather than the stromal-vascular cells, were the primary source of VEGF protein. An endothelial cell mitogenic assay showed that a major portion of the mitogenic activity of heparin-binding proteins and conditioned media derived from omentum was abolished by VEGF antibody. Additional studies with the transcription inhibitor actinomycin-D furthermore demonstrated that the VEGF gene was continuously transcribed in the rat omental adipocytes. Incubation of the omental adipocytes under hypoxic conditions induced approximately a 1.7-fold increase in VEGF protein expression, which was abolished by actinomycin-D. Northern blot analysis demonstrated that hypoxia resulted in upregulation of the VEGF mRNA in the hypoxia-cultured omental adipocytes, suggesting that the augmentation of VEGF expression in omental adipocytes by hypoxia occurs at the transcriptional level. These data suggest that VEGF is the major angiogenic factor produced by omentum and possibly underlies the mechanism of omentum-induced angiogenesis. Augmented expression of VEGF by omental cells under hypoxic conditions may furthermore reflect the mechanism responsible for enhancing the angiogenic activity of omentum in the setting of ischemia.