Mast cells (MCs) play a central role in the pathophysiology of allergic disorders through their immunomediatory secretory activity in response to IgE-dependent activation. The phosphoinositide 3-kinase (PI3K) is important for MC proliferation, adhesion and migration, allergen-IgE-induced degranulation and cytokine release. PI3K-mediated signaling also participates in the regulation of ion channels. IgE-dependent activation of MCs is mediated by an influx of extracellular Ca2+ , which is sensitive to cell membrane potential and thus to K+ - and Cl- channel activity. The present study explored whether MC ion channels are regulated by PI3K. MCs were isolated from murine bone marrow and ion channel activity was determined by patch clamp. MCs express Ca2+ -activated K+ channels that are activated by IgE-antigen or the Ca2+ ionophore ionomycin, and inhibited by clotrimazole (1.5 mM). Channel activation, in response to stimulation via IgE-antigen or ionomycin, was markedly decreased by inhibition of PI3K with LY-294002 (10 mM). In addition, degranulation, measured by beta- hexosaminidase release, was significantly diminished by PI3K inhibition or by the channel inhibitor clotrimazole. The results point to a role for PI3K-dependent Ca2+ -activated K+ channels in the regulation of MC function.