Mast cells play critical roles in allergic responses. Calcium signaling controls the function of these cells, and a role for actin in regulating calcium influx into cells has been suggested. We have previously identified the actin reorganizing protein Drebrin as a target of the immunosuppressant 3, 5-bistrifluoromethyl pyrazole, which inhibits calcium influx into cells. In this study, we show that Drebrin−/− mice exhibit reduced IgE-mediated histamine release and passive systemic anaphylaxis, and Drebrin−/− mast cells also exhibit defects in FcεRI-mediated degranulation. Drebrin−/− mast cells exhibit defects in actin cytoskeleton organization and calcium responses downstream of the FcεRI, and agents that relieve actin reorganization rescue mast cell FcεRI-induced degranulation. Our results indicate that Drebrin regulates the actin cytoskeleton and calcium responses in mast cells, thus regulating mast cell function in vivo.