Although the importance of the vascular endothelial growth factor (VEGF)/VEGF tyrosine kinase receptor (VEGFR) system in angiogenesis is well established, very little is known about the regulation of VEGFR expression in vascular endothelial cells. We have cloned partial cDNAs encoding bovine VEGFR-1 (flt) and -2 (flk-1) and used them to study VEGFR expression by bovine microvascular- and large vessel-derived endothelial cells. Both cell lines express flk-1, but not flt. Transforming growth factor β1 (TGF-β1) reduced the high affinity ¹²⁵I-VEGF binding capacity of both cell types in a dose-dependent manner, with a 2.0-2.7-fold decrease at 1-10 ng/ml. Cross-linking experiments revealed a decrease in ¹²⁵I-VEGF binding to a cell surface monomeric protein corresponding to Flk-1 on the basis of its affinity for VEGF, molecular mass (185-190 kDa), and apparent internalization after VEGF binding. Immunoprecipitation and Western blot experiments demonstrated a decrease in Flk-1 protein expression, and TGF-β1 reduced flk-1 mRNA levels in a dose-dependent manner. These results imply that TGF-β1 is a major regulator of the VEGF/Flk-1 signal transduction pathway in endothelial cells.