We have previously reported the existence of a synergistic interaction between vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) in the induction of angiogenesisin vitro.Here we demonstrate that bFGF increases VEGF receptor-2 (VEGFR-2/Flk-1) expression: mRNA levels were increased by 4.5- to 8.0-fold and total protein by 2.0- to 3.5-fold, in bovine microvascular endothelial (BME), aortic endothelial (BAE), and transformed fetal aortic (GM7373) endothelial cells. VEGF itself did not affect VEGFR-2 expression, and neither bFGF nor VEGF altered expression of FGF receptor-1. We also show that synergism occurs at the level of proliferation when this is measured in a three-dimensional but not in a conventional two-dimensional assay. Differences in the level of VEGFR-2 expression were also observed when cells were grown on or within collagen gels under different conditions: mRNA levels were lowest under sparse conditions, increased 20- to 26-fold at confluence, and increased even further (57-fold) when cells were cultured in suspension in three-dimensional collagen gels. Finally, a synergistic increase was seen in the level of expression of urokinase and urokinase receptor mRNAs when cells were exposed to bFGF and VEGF for 4 days. These findings demonstrate that the level of VEGFR-2 expression can be modulated by environmental factors including cytokines and the geometry of the culture conditions and provide some insight into the mechanisms of synergism between bFGF and VEGF in the induction of angiogenesisin vitro.