Mast cell activation induced by aggregation of FcεRI receptors with immunoglobulin E and antigen is mediated through the activation of multiple protein kinase cascades. Here we report that the regulatory protein RabGEF1 bound to Ras and negatively regulated Ras activation and its 'downstream' effector pathways in FcεRI-dependent mast cell activation. RabGEF1-deficient mast cells showed enhanced degranulation and release of lipid mediators and cytokines in response to FcεRI aggregation. RabGEF1-deficient mice developed severe skin inflammation and had increased numbers of mast cells. Thus, RabGEF1 is a negative regulator of FcεRI-dependent mast cell activation, and a lack of RabGEF1 results in the development of skin inflammation in vivo.