Mast-cell activation mediated by the high-affinity receptor for IgE (FcεRI) is considered to be a key event in the allergic inflammatory response. However, in a physiological setting, other receptors, such as KIT, might also markedly influence the release of mediators by mast cells. Recent studies have provided evidence that FcεRI-dependent degranulation is regulated by two complementary signalling pathways, one of which activates phospholipase Cγ and the other of which activates phosphatidylinositol 3-kinase, using specific transmembrane and cytosolic adaptor molecules. In this Review, we discuss the evidence for these interacting pathways and describe how the capacity of KIT, and other receptors, to influence FcεRI-dependent mast-cell-mediator release might be a function of the relative abilities of these receptors to activate these alternative pathways.