Tumors depend upon angiogenesis for growth and metastasis. It is therefore critical to understand the inhibitory signaling mechanisms in endothelial cells that control angiogenesis. Epac is a cyclic adenosine 5'-monophosphate-activated guanine nucleotide exchange factor for Rap1. In this study, we show that activation of Epac or Rap1 leads to potent inhibition of angiogenesis in vivo. Epac/Rap1 activation down-regulates inhibitor of differentiation 1 (Id1), which negatively regulates thrombospondin-1 (TSP1), an inhibitor of angiogenesis. Consistent with this mechanism, activation of Epac/Rap 1 induces expression of TSP1; conversely, depletion of Epac reduces TSP1 levels in endothelial cells. Blockade of TSP1 binding to its receptor, CD36, rescues inhibition of chemotaxis or angiogenesis by activated Epac/Rap1. Mitogen-activated protein kinase kinase 5, a downstream mediator of vascular endothelial growth factor, antagonizes the effects of Epac/Rap1 by inducing Id1 and suppressing TSP1 expression. Finally, TSP1 is also secreted by fibroblasts in response to Epac/Rap1 activation. These results identify Epac and Rap1 as inhibitory regulators of the angiogenic process, implicate Id1 and TSP1 as downstream mediators of Epac/Rap1, and highlight a novel interplay between pro- and antiangiogenic signaling cascades involving multiple cell types within the angiogenic microenvironment.