The mouse Flk1 (also called Kdr or Vegf-r2) gene encodes a receptor for VEGF-A. Flk1 is expressed in endothelial cells of the developing embryo. Recent studies have shown that Flk1 is expressed by multi-potent mesodermal progenitors, which give rise to various hematopoietic and cardiovascular cell lineages during development, and in differentiating ES cells, which may be used for cell transplantation therapy to treat cardiovascular diseases. Given its developmental and clinical importance in cardiovascular tissues, an animal model of Flk1 activity would be very useful. Here, we report the generation of Flk1-GFP BAC transgenic mice for monitoring Flk1 gene expression during development. We show that GFP expression in these mice serves as a surrogate marker for developing endothelial cells. Immunohistochemical analysis showed that the regions of expression of GFP and endogenous FLK1 largely overlap. Uniform GFP expression was observed in most endothelial cells at 8.5 dpc and thereafter. Flk1-GFP BAC transgenic mice should be useful for the study of both vascular development and pathological angiogenesis.