In midgestation mouse embryos, the aorta-gonad-mesonephros (AGM) region generates hematopoietic stem cells and definitive hematopoiesis is regulated by cell-cell interaction and signaling molecules. We showed that a Ras/mitogen-activated protein (MAP) kinase signaling-specific inhibitor and a dominant negative mutant Ras blocked the production of CD45(+) hematopoietic cells in embryonic day 11.5 AGM culture, indicating an essential role for the MAP kinase pathway in AGM hematopoiesis. Overexpression of the Ras/MAP kinase pathway regulator, Spred-2, in the AGM culture significantly reduced the number of CD45(+) cells. In contrast, production of CD45(+) cells front the AGM region of Spred-2-null mice was up-regulated as compared with wild-type littermates. Furthermore, Spred-2-deficient mice exhibited elevated hematopoietic colony formation from vascular endothelial-cadherin(+) cells. These data indicate that Spred-2 functions as a negative regulator of AGM hematopoiesis by inhibiting hematopoietic cytokine signaling.