Mast cells are key regulators in allergy and inflammation, and release histamine upon clustering of their IgE receptors. Here we demonstrate that murine mast cell responses are exacerbated in vitro and in vivo by autocrine signals through G protein-coupled receptors (GPCRs) and require functional phosphoinositide 3-kinase gamma (PI3Kgamma). Adenosine, acting through the A(3) adenosine receptor (A(3)AR) as well as other agonists of G(alphai)-coupled GPCRs, transiently increased PtdIns(3,4,5)P(3) exclusively via PI3Kgamma. PI3Kgamma-derived PtdIns(3,4,5)P(3) was instrumental for initiating a sustained influx of external Ca(2+) and degranulation. Mice lacking PI3Kgamma did not form edema after intradermal injection of adenosine and when challenged by passive systemic anaphylaxis. PI3Kgamma thus relays inflammatory signals through various G(i)-coupled receptors and is central to mast cell function.