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Mast cells play a central role in immediate type hypersensitivity and inflammatory events. Activation of mast cells not only can result in the release of preformed granule-associated mediators generally followed by de novo synthesis of lipid-derived substances. In the present study, we show that mast cell can be activated to release lipid mediators in absence of granule exocytosis. Primary cultured murine mast cells were stimulated with substance P and produced leukotriene C4, and prostaglandin D2 without the release of the granule-associated enzyme β-hexosaminidase. Indomethacin and nordihydroguaiaretic acid caused complete inhibition of arachidonic metabolite generation. Leukotriene C4 and prostaglandin D2 production was blocked by genistein, a specific inhibitor of tyrosine kinases, and bisindolylmaleimide, a protein kinase C inhibitor, indicating a role for both phosphorylation pathways in the substance P-stimulated lipid mediator production. We suggest that the cytokine microenvironment of the mast cell determines whether mast cell stimulation leads to only lipid mediator release or full activation. Analysis of granule-associated mediators only might underestimate the role of mast cell activation under (patho)physiological conditions. [Copyright &y& Elsevier]